Makes active theme button different from others

[shreyanshdwivedi]

Fixes #1341

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream master branch.
• I have added necessary documentation (if appropriate)
• Added Surge preview link

Changes proposed in this pull request:

• Applied css to the active theme button. I have changes background color of the active theme button. The color used is our theme color which is used throughout project

• https://pr-1342-fossasia-susper.surge.sh

[codecov]

Codecov Report

Merging #1342 into development will decrease coverage by 0.34%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development    #1342      +/-   ##
===============================================
- Coverage        54.89%   54.55%   -0.35%     
===============================================
  Files               51       51              
  Lines             1419     1428       +9     
  Branches           176      177       +1     
===============================================
  Hits               779      779              
- Misses             534      543       +9     
  Partials           106      106

Impacted Files	Coverage Δ
src/app/theme/theme.component.ts	13.97% <0%> (-1.5%)	⬇️

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[shreyanshdwivedi]

@AakashMallik @praveenojha33 @simsausaurabh please review :)
Coverage is decreasing because I've added function for which test will be added in PR #1340

[praveenojha33]

LGTM 👍 Please provide tests ASAP.

[shreyanshdwivedi]

@praveenojha33 please merge this, so that I can include it in my PR having tests