Kirin dark mode

[ben]

@kirincorleone suggested a browser-plugin hack on Discord, this is an attempt to make it a theme in the box.

• Copy-paste in some stuff
• Make it better
• Maybe reboot and finish what @rsek started with CSS vars
• Update CHANGELOG.md

[ben]

Looks like the original relies on some styles from Darkreader. I wasn't able to find a CSS file I could copy-paste straight away, but maybe this is a good time to start thinking about reorganizing our styles into something that's easily overridable. We're halfway there with the CSS vars, but it'd be nice to do the whole thing.

[rsek]

couple points:

• there's built-in flag for dark-mode. unless having it as a brand-new class enables some necessary functionality, it's probably a good idea to use the built-in flag instead. then it'll actually obey people's OS settings rather than requiring recourse to browser extensions (but it's still a good idea to include an option to override it -- people might want to style their OS dark and their sheet light, or vice versa)
• ideally, being sensitive to dark mode is a property of a theme's colour scheme, independent of a theme's layout; in other words, a variation of a single theme, rather than a discrete theme in and of itself. this is part of the point of doing a bunch of CSS vars, and trying to build in some separation of concerns w/r/t layout vs. colourways. that seperation of concerns probably won't land with my initial PR, tho
• after the initial CSS refactor PR, this could be a 25-line style sheet that sets a bunch of vars rather than a 225-line style sheet that calls out a bunch of stuff individually
• "reboot" implies the refactor has ceased :) it's more accurate to say i can only tolerate doing that job for so long before doing something else, because it's long, involved, touches a lot of files, and is generally kind of tedious

so, as far as implications on that other draft PR... TBH, i'd probably be gutting this like i am everything else. if this new theme were committed, the main work it would add on my end is scratch-writing a theme to replace it (so as not remove functionality).

[ben]

Yeah, this was meant to be a spike. I forgot about your draft PR (#490), we should rebase it and get it merged in, because there's a lot of good stuff in there.

And yeah, each theme should ideally provide a light and dark variant: Ironsworn would be black-text-on-white and light-gray-text-on-black, and Starforged would be black-text-on-teal and teal-text-on-black.