diff --git a/mod/fbrowser.php b/mod/fbrowser.php
index 81284d6b91..60b290850a 100644
--- a/mod/fbrowser.php
+++ b/mod/fbrowser.php
@@ -47,7 +47,7 @@ function fbrowser_content(App $a)
 	}
 
 	// Needed to match the correct template in a module that uses a different theme than the user/site/default
-	$theme = Strings::sanitizeFilePathItem($_GET['theme'] ?? null);
+	$theme = Strings::sanitizeFilePathItem($_GET['theme'] ?? '');
 	if ($theme && is_file("view/theme/$theme/config.php")) {
 		$a->setCurrentTheme($theme);
 	}
diff --git a/src/Console/Addon.php b/src/Console/Addon.php
index 1389e97bf4..74b0818100 100644
--- a/src/Console/Addon.php
+++ b/src/Console/Addon.php
@@ -116,8 +116,7 @@ HELP;
 	/**
 	 * Lists plugins
 	 *
-	 * @return int Return code of this command
-	 *
+	 * @return int|bool Return code of this command, false on error (?)
 	 * @throws \Exception
 	 */
 	private function list()
@@ -165,10 +164,9 @@ HELP;
 	 * Enables an addon
 	 *
 	 * @return int Return code of this command
-	 *
 	 * @throws \Exception
 	 */
-	private function enable()
+	private function enable(): int
 	{
 		$addonname = $this->getArgument(1);
 
@@ -190,10 +188,9 @@ HELP;
 	 * Disables an addon
 	 *
 	 * @return int Return code of this command
-	 *
 	 * @throws \Exception
 	 */
-	private function disable()
+	private function disable(): int
 	{
 		$addonname = $this->getArgument(1);
 
diff --git a/src/Core/Addon.php b/src/Core/Addon.php
index f6d0d49740..37ef335a47 100644
--- a/src/Core/Addon.php
+++ b/src/Core/Addon.php
@@ -124,7 +124,7 @@ class Addon
 	 * @return void
 	 * @throws \Exception
 	 */
-	public static function uninstall($addon)
+	public static function uninstall(string $addon)
 	{
 		$addon = Strings::sanitizeFilePathItem($addon);
 
@@ -149,7 +149,7 @@ class Addon
 	 * @return bool
 	 * @throws \Exception
 	 */
-	public static function install($addon)
+	public static function install(string $addon): bool
 	{
 		$addon = Strings::sanitizeFilePathItem($addon);
 
@@ -185,6 +185,8 @@ class Addon
 
 	/**
 	 * reload all updated addons
+	 *
+	 * @return void
 	 */
 	public static function reload()
 	{
@@ -222,7 +224,7 @@ class Addon
 	 * @return array with the addon information
 	 * @throws \Exception
 	 */
-	public static function getInfo($addon)
+	public static function getInfo(string $addon): array
 	{
 		$addon = Strings::sanitizeFilePathItem($addon);
 
@@ -287,7 +289,7 @@ class Addon
 	 * @param string $addon
 	 * @return boolean
 	 */
-	public static function isEnabled($addon)
+	public static function isEnabled(string $addon): bool
 	{
 		return in_array($addon, self::$addons);
 	}
@@ -297,7 +299,7 @@ class Addon
 	 *
 	 * @return array
 	 */
-	public static function getEnabledList()
+	public static function getEnabledList(): array
 	{
 		return self::$addons;
 	}
@@ -308,7 +310,7 @@ class Addon
 	 * @return array
 	 * @throws \Exception
 	 */
-	public static function getVisibleList()
+	public static function getVisibleList(): array
 	{
 		$visible_addons = [];
 		$stmt = DBA::select('addon', ['name'], ['hidden' => false, 'installed' => true]);
diff --git a/src/Core/Hook.php b/src/Core/Hook.php
index 89e882e35e..f2fb52f5e8 100644
--- a/src/Core/Hook.php
+++ b/src/Core/Hook.php
@@ -49,6 +49,8 @@ class Hook
 
 	/**
 	 * Load hooks
+	 *
+	 * @return void
 	 */
 	public static function loadHooks()
 	{
@@ -69,8 +71,9 @@ class Hook
 	 * @param string $hook
 	 * @param string $file
 	 * @param string $function
+	 * @return void
 	 */
-	public static function add($hook, $file, $function)
+	public static function add(string $hook, string $file, string $function)
 	{
 		if (!array_key_exists($hook, self::$hooks)) {
 			self::$hooks[$hook] = [];
@@ -90,7 +93,7 @@ class Hook
 	 * @return mixed|bool
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function register($hook, $file, $function, $priority = 0)
+	public static function register(string $hook, string $file, string $function, int $priority = 0)
 	{
 		$file = str_replace(DI::app()->getBasePath() . DIRECTORY_SEPARATOR, '', $file);
 
@@ -111,7 +114,7 @@ class Hook
 	 * @return boolean
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function unregister($hook, $file, $function)
+	public static function unregister(string $hook, string $file, string $function): bool
 	{
 		$relative_file = str_replace(DI::app()->getBasePath() . DIRECTORY_SEPARATOR, '', $file);
 
@@ -120,8 +123,8 @@ class Hook
 		self::delete($condition);
 
 		$condition = ['hook' => $hook, 'file' => $relative_file, 'function' => $function];
-		$result = self::delete($condition);
-		return $result;
+
+		return self::delete($condition);
 	}
 
 	/**
@@ -130,7 +133,7 @@ class Hook
 	 * @param  string $name Name of the hook
 	 * @return array
 	 */
-	public static function getByName($name)
+	public static function getByName(string $name): array
 	{
 		$return = [];
 
@@ -149,9 +152,10 @@ class Hook
 	 * @param integer $priority of the hook
 	 * @param string  $name     of the hook to call
 	 * @param mixed   $data     to transmit to the callback handler
+	 * @return void
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function fork($priority, $name, $data = null)
+	public static function fork(int $priority, string $name, $data = null)
 	{
 		if (array_key_exists($name, self::$hooks)) {
 			foreach (self::$hooks[$name] as $hook) {
@@ -184,9 +188,10 @@ class Hook
 	 *
 	 * @param string        $name of the hook to call
 	 * @param string|array &$data to transmit to the callback handler
+	 * @return void
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function callAll($name, &$data = null)
+	public static function callAll(string $name, &$data = null)
 	{
 		if (array_key_exists($name, self::$hooks)) {
 			foreach (self::$hooks[$name] as $hook) {
@@ -202,9 +207,10 @@ class Hook
 	 * @param string          $name of the hook to call
 	 * @param array           $hook Hook data
 	 * @param string|array   &$data to transmit to the callback handler
+	 * @return void
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function callSingle(App $a, $name, $hook, &$data = null)
+	public static function callSingle(App $a, string $name, array $hook, &$data = null)
 	{
 		// Don't run a theme's hook if the user isn't using the theme
 		if (strpos($hook[0], 'view/theme/') !== false && strpos($hook[0], 'view/theme/' . $a->getCurrentTheme()) === false) {
@@ -229,7 +235,7 @@ class Hook
 	 * @param string $name Name of the addon
 	 * @return boolean
 	 */
-	public static function isAddonApp($name)
+	public static function isAddonApp(string $name): bool
 	{
 		$name = Strings::sanitizeFilePathItem($name);
 
@@ -253,7 +259,7 @@ class Hook
 	 * @return bool
 	 * @throws \Exception
 	 */
-	public static function delete(array $condition)
+	public static function delete(array $condition): bool
 	{
 		$result = DBA::delete('hook', $condition);
 
@@ -273,7 +279,7 @@ class Hook
 	 * @return bool
 	 * @throws \Exception
 	 */
-	private static function insert(array $condition)
+	private static function insert(array $condition): bool
 	{
 		$result = DBA::insert('hook', $condition);
 
diff --git a/src/Core/L10n.php b/src/Core/L10n.php
index 050c190737..08296068bb 100644
--- a/src/Core/L10n.php
+++ b/src/Core/L10n.php
@@ -128,7 +128,7 @@ class L10n
 	private function setLangFromSession(IHandleSessions $session)
 	{
 		if ($session->get('language') !== $this->lang) {
-			$this->loadTranslationTable($session->get('language'));
+			$this->loadTranslationTable($session->get('language') ?? $this->lang);
 		}
 	}
 
@@ -140,10 +140,10 @@ class L10n
 	 * Uses an App object shim since all the strings files refer to $a->strings
 	 *
 	 * @param string $lang language code to load
-	 *
+	 * @return void
 	 * @throws \Exception
 	 */
-	private function loadTranslationTable($lang)
+	private function loadTranslationTable(string $lang)
 	{
 		$lang = Strings::sanitizeFilePathItem($lang);
 
@@ -183,7 +183,7 @@ class L10n
 	 *
 	 * @return string The two-letter language code
 	 */
-	public static function detectLanguage(array $server, array $get, string $sysLang = self::DEFAULT)
+	public static function detectLanguage(array $server, array $get, string $sysLang = self::DEFAULT): string
 	{
 		$lang_variable = $server['HTTP_ACCEPT_LANGUAGE'] ?? null;
 
@@ -269,7 +269,7 @@ class L10n
 	 *
 	 * @return string
 	 */
-	public function t($s, ...$vars)
+	public function t(array $s, ...$vars): string
 	{
 		if (empty($s)) {
 			return '';
@@ -307,7 +307,7 @@ class L10n
 	 * @return string
 	 * @throws \Exception
 	 */
-	public function tt(string $singular, string $plural, int $count)
+	public function tt(string $singular, string $plural, int $count): string
 	{
 		$s = null;
 
@@ -352,7 +352,7 @@ class L10n
 	 *
 	 * @return bool
 	 */
-	private function stringPluralSelectDefault($n)
+	private function stringPluralSelectDefault(int $n): bool
 	{
 		return $n != 1;
 	}
@@ -369,7 +369,7 @@ class L10n
 	 *
 	 * @return array
 	 */
-	public static function getAvailableLanguages()
+	public static function getAvailableLanguages(): array
 	{
 		$langs              = [];
 		$strings_file_paths = glob('view/lang/*/strings.php');
@@ -391,10 +391,9 @@ class L10n
 	 * Translate days and months names.
 	 *
 	 * @param string $s String with day or month name.
-	 *
 	 * @return string Translated string.
 	 */
-	public function getDay($s)
+	public function getDay(string $s): string
 	{
 		$ret = str_replace(['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'],
 			[$this->t('Monday'), $this->t('Tuesday'), $this->t('Wednesday'), $this->t('Thursday'), $this->t('Friday'), $this->t('Saturday'), $this->t('Sunday')],
@@ -411,10 +410,9 @@ class L10n
 	 * Translate short days and months names.
 	 *
 	 * @param string $s String with short day or month name.
-	 *
 	 * @return string Translated string.
 	 */
-	public function getDayShort($s)
+	public function getDayShort(string $s): string
 	{
 		$ret = str_replace(['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'],
 			[$this->t('Mon'), $this->t('Tue'), $this->t('Wed'), $this->t('Thu'), $this->t('Fri'), $this->t('Sat'), $this->t('Sun')],
@@ -435,7 +433,7 @@ class L10n
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 * @hook poke_verbs pokes array
 	 */
-	public function getPokeVerbs()
+	public function getPokeVerbs(): array
 	{
 		// index is present tense verb
 		// value is array containing past tense verb, translation of present, translation of past
@@ -461,7 +459,7 @@ class L10n
 	 * @return static A new L10n instance
 	 * @throws \Exception
 	 */
-	public function withLang(string $lang)
+	public function withLang(string $lang): L10n
 	{
 		// Don't create a new instance for same language
 		if ($lang === $this->lang) {
diff --git a/src/Core/Theme.php b/src/Core/Theme.php
index 57dfaa4d63..834787da69 100644
--- a/src/Core/Theme.php
+++ b/src/Core/Theme.php
@@ -32,7 +32,7 @@ require_once 'boot.php';
  */
 class Theme
 {
-	public static function getAllowedList()
+	public static function getAllowedList(): array
 	{
 		$allowed_themes_str = DI::config()->get('system', 'allowed_themes');
 		$allowed_themes_raw = explode(',', str_replace(' ', '', $allowed_themes_str));
@@ -69,7 +69,7 @@ class Theme
 	 * @param string $theme the name of the theme
 	 * @return array
 	 */
-	public static function getInfo($theme)
+	public static function getInfo(string $theme): array
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -133,7 +133,7 @@ class Theme
 	 * @return string
 	 * @throws \Friendica\Network\HTTPException\InternalServerErrorException
 	 */
-	public static function getScreenshot($theme)
+	public static function getScreenshot(string $theme): string
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -146,7 +146,13 @@ class Theme
 		return DI::baseUrl() . '/images/blank.png';
 	}
 
-	public static function uninstall($theme)
+	/**
+	 * Uninstalls given theme name
+	 *
+	 * @param string $theme Name of theme
+	 * @return bool true on success
+	 */
+	public static function uninstall(string $theme)
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -167,10 +173,18 @@ class Theme
 		if ($key !== false) {
 			unset($allowed_themes[$key]);
 			Theme::setAllowedList($allowed_themes);
+			return true;
 		}
+		return false;
 	}
 
-	public static function install($theme)
+	/**
+	 * Installs given theme name
+	 *
+	 * @param string $theme Name of theme
+	 * @return bool true on success
+	 */
+	public static function install(string $theme): bool
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -208,7 +222,7 @@ class Theme
 	 * @return string Path to the file or empty string if the file isn't found
 	 * @throws \Exception
 	 */
-	public static function getPathForFile($file)
+	public static function getPathForFile(string $file): string
 	{
 		$a = DI::app();
 
@@ -237,10 +251,9 @@ class Theme
 	 * Provide a sane default if nothing is chosen or the specified theme does not exist.
 	 *
 	 * @param string $theme Theme name
-	 *
 	 * @return string
 	 */
-	public static function getStylesheetPath($theme)
+	public static function getStylesheetPath(string $theme): string
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -263,10 +276,10 @@ class Theme
 	/**
 	 * Returns the path of the provided theme
 	 *
-	 * @param $theme
+	 * @param string $theme Theme name
 	 * @return string|null
 	 */
-	public static function getConfigFile($theme)
+	public static function getConfigFile(string $theme)
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);
 
@@ -285,11 +298,11 @@ class Theme
 	/**
 	 * Returns the background color of the provided theme if available.
 	 *
-	 * @param string   $theme
+	 * @param string   $theme Theme name
 	 * @param int|null $uid   Current logged-in user id
 	 * @return string|null
 	 */
-	public static function getBackgroundColor(string $theme, $uid = null)
+	public static function getBackgroundColor(string $theme, int $uid = null)
 	{
 		$theme = Strings::sanitizeFilePathItem($theme);