zen-browser: add enableCSS option

[Mikilio]

Description

userChrome.css and userContent.css are technically unsupported, but still a quite popular form of customizing the UI of Firefox and forks.

In the case of Zen Browser, however, it gets in the way of per workspace theming, which is a very desirable feature.
This is why even though it concerns all Firefox forks, I am mentioning it here.

The approved way to modify Firefox and its forks is through extensions, and in the future stylix should probably just build a Firefox theme.

For now, I believe it could improve UX if you could just disable userChrome userContent and the preference enabling this using a single flag.

Submission Checklist

• I assert that this issue is relevant to Stylix
• I assert that this is not a duplicate of an existing issue

Notify maintainers

@MrSom3body @Flameopathic @danth @trueNAHO

[permafrozen]

I researched a bit and I don't think this makes a lot of sense, but maybe I misinterpreted something so let me know.

Pros

• more stable than userChrome.css/userContent.css
  theoretically less theming work
• The way mozilla wants you to theme firefox

Cons

• zen doesn't support firefox themes
  or atleast I couldn't figure it out
  and after googling once, other users said the same

• stylix would have to regenerate the extension on each build
  the only easy way to load it is via the debugging feature temporary extensions
  the extension would therefore need to be loaded each time a new session gets loaded

  but this isn't the biggest problem
  since the zen-browser flake also somehow loads extensions
  haven't looked into this, if it also works with theme add-ons and if they are the same

• to create/publish these extensions you need a mozilla account
  if it shouldn't be loaded locally

• A lot more limiting, because of that not really adopted in the firefox community

Conclusion

Yeah I don't think this is possible to implement
at this stage of the firefox add-on themes

Maybe I overlooked something
but there is generally not a lot of information about this type of theming
since most users prefer the legacy userChrome variant of theming

[Mikilio]

So basically Zen did create settings to select themes and currently ships with something they call gradient theme. This is a theme that works on a per workspace basis and is on by default interfeering with some of the userChrome installed by stylix.

The recommended flake (by the creator of zen) also makes a hm module to configure exactly that.
https://github.com/0xc000022070/zen-browser-flake

I was proposing a switch off flag for zen only just so that it doesn't interfere with zen supplied themes ( currently only one but still)

Of course I can always just mkForce whatever I want but I suspect this to be a more common override in the future.

[permafrozen]

Oh okay, sorry for misunderstanding, this makes sense.