Add theme selector in settings

[FredGuiou]

@fraxken Here is this WIP to implement a theme management. The theme is configurable in settings. Could you please make a first review ?

[fraxken]

Warnings in home page probably need a bit of work:

separator and title too:

packages navigation doesn't pair well in dark

search page:

[PierreDemailly]

The legend is broken with dark theme because FRIENDLY is missing (only available with light theme).

cli/workspaces/vis-network/src/constants.js

Line 42 in 73b8313

FRIENDLY: {

[PierreDemailly]

For the search page, i think the color when hovering items could be fine as default (top is hover and bottom is default on the following screen)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.78%. Comparing base (28ee6a9) to head (fd467e2).
Report is 190 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##            master     #494       +/-   ##
============================================
- Coverage   100.00%   83.78%   -16.22%     
============================================
  Files           15       29       +14     
  Lines          649     1973     +1324     
  Branches         0        1        +1     
============================================
+ Hits           649     1653     +1004     
- Misses           0      320      +320     

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