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/manage_locations.html

@@ -78,12 +78,12 @@
         border-radius: 5px;
         box-shadow: 0 2px 5px rgba(0,0,0,0.1);
         margin-bottom: 20px;
-        background-color: #fff;
+        background-color: var(--ui-surface);
         border: 1px solid #e9ecef;
     }
     
     .card-header {
-        background-color: #f8f9fa;
+        background-color: var(--ui-surface-soft);
         padding: 15px 20px;
         border-bottom: 1px solid #e9ecef;
     }
@@ -170,7 +170,7 @@
         justify-content: space-between;
         align-items: center;
         padding: 10px;
-        background-color: #f8f9fa;
+        background-color: var(--ui-surface-soft);
         border-radius: 4px;
         border-left: 3px solid #6c757d;
     }