feat: add background theming

[alexdln]

I've set up theme customization. I've created a new hook to avoid using overly complex settings in the root script. I've also added a root script to prevent flickering.

I did all the customization using css-vars and left the default theme as is (it will also be used if nothing is selected or there are no variables for some reason).

I've tried not to break the current patterns, but if you have any questions, let me know - I'll fix.

If everything is okay, I need help with translations and copyright in general (I'm not sure what the best name for this setting is).

Closes #662

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Project	Deployment	Actions	Updated (UTC)
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[danielroe]

this is very nice 👌

[github-actions]

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