feat(webui): add darkmode theme

[tsirysndr]

#114

[codecov]

Codecov Report

Base: 47.42% // Head: 47.40% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (439bf5f) compared to base (f23e4cc).
Patch has no changes to coverable lines.

❗ Current head 439bf5f differs from pull request most recent head 86986c7. Consider uploading reports for the commit 86986c7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   47.42%   47.40%   -0.03%     
==========================================
  Files         104      104              
  Lines       12725    12725              
==========================================
- Hits         6035     6032       -3     
- Misses       6690     6693       +3     

Impacted Files	Coverage Δ
storage/src/repo/artist.rs	97.67% <0.00%> (-2.33%)	⬇️
server/src/library.rs	66.66% <0.00%> (-1.67%)	⬇️
storage/src/repo/track.rs	98.41% <0.00%> (-1.59%)	⬇️
graphql/src/schema/library.rs	46.86% <0.00%> (ø)
graphql/src/schema/playlist.rs	64.65% <0.00%> (ø)
storage/src/repo/playlist.rs	37.63% <0.00%> (+1.07%)	⬆️

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