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/student_card_barcode_print.html

@@ -45,7 +45,7 @@
         }
 
         h1 {
-            color: #333;
+            color: var(--ui-text);
             font-size: 28px;
             margin-bottom: 10px;
         }
@@ -67,7 +67,7 @@
         }
 
         .info-box strong {
-            color: #333;
+            color: var(--ui-text);
         }
 
         .info-box p {
@@ -106,7 +106,7 @@
 
         .btn-secondary {
             background: #e0e0e0;
-            color: #333;
+            color: var(--ui-text);
         }
 
         .btn-secondary:hover {