refactor(src/shared/styles): replace hardcoded CSS values with design token variables

Introduce consistent design tokens for radii and colors, replacing alpha() and hardcoded values with theme variables for easier theming and maintenance.

- New Features:   - Add CSS variable usage for radii (var(--radius-*)) and color tokens (@border-subtle, @text-main, @accent)
- Enhancements:
  - Replace hardcoded border, radius, and color definitions with variable references
  - Align styling with the project's theme system
- Bug Fixes:
  - No bug fixes included

src/shared/styles/switcher.css

@@ -1,6 +1,6 @@
 #application-switcher-container {
     background-color: var(--shadow);
-    border-radius: 20px;
+    border-radius: var(--radius-xl);
     padding: 24px;
 }
 
@@ -17,7 +17,7 @@
 
 #window-button {
     background-color: transparent;
-    border-radius: 12px;
+    border-radius: var(--radius-md);
 }
 
 #window-button.active {
@@ -39,7 +39,7 @@
 }
 
 #switcher-icon-box {
-    border-radius: 12px;
+    border-radius: var(--radius-md);
     padding: 4px;
 }