[chore/themes] Add auto-switching themes for blurple/brutalist/solarized

[cdn0x12]

Description

Added auto-switching variants for themes that have both light and dark versions (blurple, brutalist, and solarized). These new variants use prefers-color-scheme media query to automatically switch between light and dark themes based on user's system preferences, as discussed in #1214 .

Each auto theme defaults to its dark version if prefers-color-scheme is not supported by the browser, while brutalist default to brutalist.css(the light version).

The auto-switching theme uses import to load the corresponding light/dark version themes. This way, in the future, they'll automatically follow the update, thus barely add any maintenance workload.

TODO

If this PR meets expectations and gets merged, the next step is to

• create a light version of the base CSS for GoToSocial
• add a theme switch button.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Brilliant stuff :) Simple and elegant. Thank you very much!