Use data-theme attribute for theme switching and set color-scheme (#1808)

- Update theme toggle and initialization scripts to set data-theme attribute
- Replace CSS selectors using .dark class with [data-theme="dark"] and 
  [data-theme="light"] attribute selectors
- Set CSS color-scheme variable alongside theme colors for better native 
  control appearance
- Removes redundant styles

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Co-authored-by: Aditya Telange <21258296+adityatelange@users.noreply.github.com>

assets/css/common/header.css

@@ -41,12 +41,11 @@ button#theme-toggle {
     margin: auto 4px;
 }
 
-body.dark #moon {
-    vertical-align: middle;
+[data-theme="dark"] #moon {
     display: none;
 }
 
-body:not(.dark) #sun {
+[data-theme="light"] #sun {
     display: none;
 }