In his never-ending quest to keep improving Tildes's theme system, @Bauke has reworked a major portion of it again, this time making it use CSS custom properties (instead of pre-generating a lot of theme-specific rules using Sass).
This new method has a lot of benefits, including reducing the size of Tildes's CSS file to less than half of what it was before. It will also make it much simpler for people to override certain site colors or create their own themes using local CSS changes (e.g. through extensions like Stylus). (Note: please don't invest a lot of time into using it to customize yet, since it may still need to have further changes or even reverted)
This is a relatively modern CSS feature that should have good support at this point, but it's possible there will still be some issues, or things that were missed during the conversion. If you notice any colors being wrong or other appearance changes (even minor ones), please leave a comment (including which theme you're using) so that I can fix them up.
Thanks yet again, @Bauke!
And I haven't done it in a while now, but I've topped everyone back up to 10 invites, accessible on the invite page.