Refactor UI styles to use CSS variables for consistent theming

- Updated background colors across various templates to use CSS variables for better theming support.
- Replaced hardcoded colors with variables like `var(--ui-surface)` and `var(--ui-surface-soft)` for backgrounds.
- Adjusted text colors to use `var(--ui-text)` for improved readability and consistency.
- Enhanced the theme toggle functionality in the base template to dynamically change theme colors based on user preference.
- Ensured that all relevant templates reflect the new theming approach for a cohesive user experience.

Web/templates/user_del.html

@@ -407,14 +407,14 @@
     }
     
     .filter-bar {
-        background-color: #f8f9fa;
+        background-color: var(--ui-surface-soft);
         padding: 15px;
         border-radius: 6px;
         border: 1px solid #dee2e6;
     }
 
     .password-requirements {
-        background-color: #f8f9fa;
+        background-color: var(--ui-surface-soft);
         padding: 10px;
         border-radius: 4px;
         margin-top: 10px;
@@ -439,7 +439,7 @@
         border: 1px solid #dee2e6;
         border-radius: 8px;
         padding: 12px;
-        background: #f8fafc;
+        background: var(--ui-surface-soft);
     }
 
     .permissions-group h6 {