feat: community style snippets

[ArjixWasTaken]

The problem

Currently, a lot of people have frustrations with the way music.youtube.com is designed, and would like changes to be done.
The issue is that, it doesn't make sense to make/update a plugin for every single user suggestion.

Especially since a lot of UI issues can be fixed by just using a custom CSS file, which is already supported by th-ch/youtube-music.

Proposal

We make one plugin that "rules them all" (when it comes to CSS).
It would be a plugin that allows users to apply CSS snippets shared amongst the community,
similar to BetterDiscord's Themes, but focused on small UI tweaks instead of full client themes.

That way, issues like #2539, would not exist in the first place.

Specifics

As for how we would go about doing this, there are two routes we can take.

1. Host user-themes on a ^*central github repo, so users would need to make a PR in order to add a tweak.
2. We host a backend where users can sign into and submit CSS snippets directly from within the app.

No matter what route we take, we should inspect each and every CSS snippet, to avoid stuff like key loggers or IP loggers.

Notes

A big benefit to this would be that non-technical users would be able to use a plethora of appearance tweaks that we would not have included otherwise.

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*: By that I mean a dedicated repo, so that th-ch/youtube-music doesn't have to release an update for each and every new snippet

[ArjixWasTaken]

CC: @th-ch, @JellyBrick, @Su-Yong

[ArjixWasTaken]

Example from a cousin project spicetify: