Refactor CSS files in src/screens/Leaderboard

[AVtheking]

Refactor CSS files in src/screens/Leaderboard

To improve the UI/UX of Talawa-Admin we have been working on:

1. making the application more suitable to color blind users
2. streamlining all CSS into a single global file

Additional information:

1. The foundational work for this was completed in this PR:
   • Chore/css changes #2466
2. The desired single CSS file is:
   • src/style/app.module.css

The challenge:

The goal is to convert the CSS file in this subdirectory and all the components related to this screen to use this new design pattern. This means that:

1. All CSS files in this subdirectory and all the CSS files in related components must be merged into the global CSS file using pre-existing CSS.
2. All CSS files in this subdirectory and all the CSS files in related components must be deleted after the merge.
3. The remaining relevant files in this subdirectory must only reference the global CSS file.
4. All the CSS files in the related components of this subdirectory must adhere to the same guidelines.

[github-actions]

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