[UI] Add basic themes support

[arielj]

This PR adds basic themes support with a few hardcoded themes.

themes2.mp4

I removed a few css variables there were not being used, I didn't do a refactor on the variables (it could be nice to improve how we use the variables but it was not really needed for this PR)

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[arielj]

since modal backdrop can't use css variables, this is a workaround using a box-shadow that covers the whole screen

[flavioislima]

Tested here and looks pretty great 💯
The only thing is that we need to check with Billie if the colors on the game page (install, settings, etc) are ok.
I think there is one mistake on our current theme that we are using the same colors for Import and Install, I asked her to fix that.
And for the new themes, the hover colors I believe are not 100% ok as well. But lets wait until she updates the design in Figma :)

[arielj]

This is ready for review, I updated the themes based on figma (though the design don't completely match so things can be a bit off since I had to do some guessing). I also added classic versions of all with the variant on the sidebar links color since I know it's a somewhat controversial topic.

[flavioislima]

Tested it and looks fine. If we need to update something we can do it later.