refactor: reorganize CSS structure and standardize CSS variable usage for improved maintainability (#690)

* refactor: CSS variables to use fixit-var for consistency across styles

* refactor: reorganize CSS structure and consolidate imports for improved maintainability

* refactor: unify CSS variable usage and improve structure across stylesheets

* refactor: standardize CSS variable usage for layout

* refactor: standardize CSS variable usage for archive and special pages

* refactor: standardize CSS variable usage for single page

* refactor: standardize CSS variable usage for widgets

* refactor(css): refactors the border-radius mixin and adds the .rounded-full utility class

* refactor: standardize z-index usage across components

* refactor(css): removes deprecated compatibility mixins and replaces them with native CSS properties

* feat: unify color variable naming and update theme color values

- Rename `global-font-secondary-color` to `global-placeholder-color` for more accurate description of its purpose
- Update multiple style references to use the new `secondary` theme color variable
- Update theme color mapping to the new color values ​​for improved visual consistency
- Fixed the use of search box placeholder colors and removed hard-coded transparency values
- Added color preview test documentation for easier verification of color rendering
- Cleaned up some duplicate dark mode styles and replaced them with unified CSS variables

* refactor: remove the `hr-background-color` variable and uses `global-border-color` uniformly

* feat(CSE): add dark theme style to Google CSE search box and results container

* fix(task-lists): fixed styling and rendering issues with nested task lists

.github/copilot-instructions.md

@@ -90,7 +90,7 @@ FixIt/
 2. **Mixin 使用**: 提高代码复用性
 
    ```scss
-   @include border-radius($global-border-radius);
+   @include border-radius;
    @include transition(all 0.2s ease);
    @include blur;
    ```