Fix an issue where the light theme briefly appears when loading pgAdmin…

web/pgadmin/browser/__init__.py

@@ -400,12 +400,6 @@ def index():
             'pass_enc_key' in session:
         session['allow_save_password'] = False
 
-    response = Response(render_template(
-        MODULE_NAME + "/index.html",
-        username=current_user.username,
-        _=gettext
-    ))
-
     # Set the language cookie after login, so next time the user will have that
     # same option at the login time.
     misc_preference = Preferences.module('misc')
@@ -416,11 +410,22 @@ def index():
     if user_languages:
         language = user_languages.get() or 'en'
 
+    # Get the theme preference
+    user_theme = misc_preference.preference('theme')
+    theme = user_theme.get() or 'light' if user_theme else 'light'
+
     domain = dict()
     if config.COOKIE_DEFAULT_DOMAIN and\
             config.COOKIE_DEFAULT_DOMAIN != 'localhost':
         domain['domain'] = config.COOKIE_DEFAULT_DOMAIN
 
+    response = Response(render_template(
+        MODULE_NAME + "/index.html",
+        username=current_user.username,
+        theme=theme,
+        _=gettext
+    ))
+
     response.set_cookie("PGADMIN_LANGUAGE", value=language,
                         path=config.SESSION_COOKIE_PATH,
                         secure=config.SESSION_COOKIE_SECURE,

web/pgadmin/browser/templates/browser/index.html

@@ -3,6 +3,9 @@
 {% block title %}{{ config.APP_NAME }}{% endblock %}
 
 {% block init_script %}
+// Set theme on the global window object
+window.theme = "{{ theme }}";
+
 function parseConsoleArgs(args) {
   const retData = Array.from(args).map(arg => {
     try {

web/pgadmin/static/js/Theme/index.jsx

@@ -882,12 +882,34 @@ function getFinalTheme(baseTheme) {
   }, baseTheme);
 }
 
+/* Get the actual system theme is user selected system theme in preferences */
+function parseSystemTheme(selectedTheme) {
+  if (selectedTheme === 'system') {
+    const systemMatchMedia = matchMedia('(prefers-color-scheme: dark)');
+    return {
+      'theme': systemMatchMedia.matches ? 'dark' : 'light',
+      'systemMatchMedia': systemMatchMedia,
+    };
+  }
+  return {
+    'theme': selectedTheme,
+    'systemMatchMedia': null,
+  };
+}
+
 /* Theme wrapper used by DOM containers to apply theme */
 /* In future, this will be moved to App container */
 export default function Theme({children}) {
   const prefStore = usePreferences();
-  const [theme, setTheme] = useState();
+  const selectedTheme =
+    prefStore?.getPreferencesForModule('misc')?.theme ||
+    window.theme ||
+    'light';
+
+  // Initialize theme state
+  const [theme, setTheme] = useState(parseSystemTheme(selectedTheme).theme);
 
+  // Memoize the theme object
   const themeObj = useMemo(()=>{
     let baseTheme = getLightTheme(basicSettings);
     switch(theme) {
@@ -901,31 +923,26 @@ export default function Theme({children}) {
     return getFinalTheme(baseTheme);
   }, [theme]);
 
+  // Handle theme updates
   useEffect(() => {
-    const selectedTheme = prefStore.getPreferencesForModule('misc').theme;
-    if(theme && theme === selectedTheme) {
-      return;
-    }else{
-      if (selectedTheme !== 'system') {
-        setTheme(selectedTheme);
-        return;
-      }
-      const isSystemInDarkMode = matchMedia('(prefers-color-scheme: dark)');
-      setTheme(isSystemInDarkMode.matches ? 'dark' : 'light');
-      const listener = (event) => {
-        setTheme(event.matches ? 'dark' : 'light');
-      };
-      isSystemInDarkMode.addEventListener('change',listener);
-      return () => {
-        isSystemInDarkMode.removeEventListener('change',listener);
-      };
-    }
-  },[prefStore]);
+    let {theme, systemMatchMedia} = parseSystemTheme(selectedTheme);
+    setTheme(theme);
+
+    const updateTheme = (event) => {
+      const newTheme = event.matches ? 'dark' : 'light';
+      setTheme(newTheme);
+    };
+    systemMatchMedia?.addEventListener('change', updateTheme);
+
+    return () => {
+      systemMatchMedia?.removeEventListener('change', updateTheme);
+    };
+  },[selectedTheme]);
 
   return (
     <ThemeProvider theme={themeObj}>
       <CssBaseline />
-      <LocalizationProvider dateAdapter={AdapterDateFns} >
+      <LocalizationProvider dateAdapter={AdapterDateFns}>
         {children}
       </LocalizationProvider>
     </ThemeProvider>