fixes #1492: create enum for all theme types'

[sakshee-19]

Fixes #1492

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:

• Added single configuration for all theme types.

• https://pr-1494-fossasia-susper.surge.sh

[codecov]

Codecov Report

Merging #1494 into development will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           development    #1494      +/-   ##
===============================================
+ Coverage         63.4%   63.45%   +0.05%     
===============================================
  Files               51       52       +1     
  Lines             1451     1453       +2     
  Branches           181      181              
===============================================
+ Hits               920      922       +2     
+ Misses             423      420       -3     
- Partials           108      111       +3

Impacted Files	Coverage Δ
src/app/types/theme.ts	100% <100%> (ø)
src/app/theme/theme.component.ts	90.62% <50%> (+0.09%)	⬆️
src/app/intelligence/intelligence.component.ts	76% <0%> (-8%)	⬇️
src/app/services/intelligence.service.ts	83.33% <0%> (+8.33%)	⬆️

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[simsausaurabh]

@sakshee-19 we can also convert other hard-coded enums/types.