fix: fixed hover color issue throughout MFE

[sundasnoreen12]

INF-1220

Description

1. Updated the direct color codes in scss file with paragon color variables in discussion MFE.
2. Fixed hover color issue throughout discussion MFE
3. Fixed ZIndex issue of card

How Has This Been Tested?

Screenshots/sandbox (optional):

Include a link to the sandbox for design changes or screenshot for before and after. Remove this section if it's not applicable.

|Before|After| Before

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Post-merge Checklist

• [ ] Deploy the changes to prod after verifying on stage or ask @openedx/edx-infinity to do it.
• [ ] 🎉 🙌 Celebrate! Thanks for your contribution.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (92adec3) 92.38% compared to head (e9efc83) 92.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #644 +/- ## ======================================= Coverage 92.38% 92.38% ======================================= Files 169 169 Lines 3452 3452 Branches 892 892 ======================================= Hits 3189 3189 Misses 243 243 Partials 20 20 ```

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