diff --git a/application/forms/PreferenceForm.php b/application/forms/PreferenceForm.php
index e03d19260..6ab2ade3a 100644
--- a/application/forms/PreferenceForm.php
+++ b/application/forms/PreferenceForm.php
@@ -213,12 +213,8 @@ class PreferenceForm extends Form
         if (isset($formData['theme']) && $formData['theme'] !== StyleSheet::DEFAULT_THEME) {
             $themeFile = StyleSheet::getThemeFile($formData['theme']);
             $file = $themeFile !== null ? @file_get_contents($themeFile) : false;
-            if ($file && ! preg_match(StyleSheet::REGEX_ALL_MODE_QUERY, $file)) {
+            if ($file && strpos($file, StyleSheet::LIGHT_MODE_IDENTIFIER) === false) {
                 $disabled = ['', 'light', 'system'];
-                if (preg_match(StyleSheet::REGEX_AUTO_MODE_QUERY, $file)) {
-                    $value = 'system';
-                    $disabled = ['', 'light'];
-                }
             }
         }
 
diff --git a/library/Icinga/Web/StyleSheet.php b/library/Icinga/Web/StyleSheet.php
index d388f5924..1033f08db 100644
--- a/library/Icinga/Web/StyleSheet.php
+++ b/library/Icinga/Web/StyleSheet.php
@@ -41,19 +41,11 @@ class StyleSheet
     ];
 
     /**
-     * RegEx pattern for matching full css @media query of theme mode
+     * Sequence that signals that a theme supports light mode
      *
      * @var string
      */
-    const REGEX_ALL_MODE_QUERY = '/@media\s*\(\s*min-height\s*:\s*@prefer-light-color-scheme\s*\)\s*,' .
-    '\s*\(\s*prefers-color-scheme\s*:\s*light\s*\)\s*and\s*\(\s*min-height\s*:\s*@enable-color-preference\s*\)/';
-
-    /**
-     * RegEx pattern for matching css @media query of theme mode
-     *
-     * @var string
-     */
-    const REGEX_AUTO_MODE_QUERY = '/@media.*prefers-color-scheme/';
+    const LIGHT_MODE_IDENTIFIER = '@light-mode:';
 
     /**
      * Array of core LESS files Web 2 sends to the client
@@ -178,12 +170,8 @@ class StyleSheet
 
         $mode = 'none';
         if ($user = Auth::getInstance()->getUser()) {
-            $file = $themePath !== null ? @file_get_contents($themePath) : '';
-            if ($file && $file !== '' && ! preg_match(self::REGEX_ALL_MODE_QUERY, $file)) {
-                if (preg_match(self::REGEX_AUTO_MODE_QUERY, $file)) {
-                    $mode = 'system';
-                }
-            } else {
+            $file = $themePath !== null ? @file_get_contents($themePath) : false;
+            if (! $file || strpos($file, self::LIGHT_MODE_IDENTIFIER) !== false) {
                 $mode = $user->getPreferences()->getValue('icingaweb', 'theme_mode', self::DEFAULT_MODE);
             }
         }