Fix accessibility issues w/ color contrast, match other styles to conventions

Hi, thanks for your hard work on such a great project! Have used it for years without ever visiting the website.

I can infer that the website is a labor of love (esp. via the themes at the bottom, very cute) but it screams "talented backend engineer does frontend", if you'll excuse the colloquialism. As the title states, I have ~~two~~ three concerns:

1. Body text on the splash page fails a "Contrast Check", which is not only a branding issue but a serious accessibility concern -- I had a coworker who would get bad headaches trying to read such text, for a concrete example. See tests for both [the winter theme](https://webaim.org/resources/contrastchecker/?fcolor=9CA3AF&bcolor=4588F6) and [the main (?) theme](https://webaim.org/resources/contrastchecker/?fcolor=9CA3AF&bcolor=008C93).

2. Similarly, the pride themes are quite wholesome, but in their current state seem to be something of an accessibility nightmare for their own reasons (on top of contrast issues). I don't have a link for this one, but mostly just because there's no accessibility rules or design principles covering text that changes colors *mid glyph* 😬

3. I think there's good, objective reasons to avoid the clearly hand-rolled buttons and headers, both for maintainability and for clarity. See, for example, point 10 here: https://principles.design/examples/the-ten-principles-of-inclusive-design

**I'd be honored to submit a pull request sometime with concrete proposals if y'all want the help**, but figured I'd check the general sentiment on the issue before putting in the effort!

Start by reviewing the splash-page contrast examples linked in the issue, then inspect how the pride themes and hand-rolled buttons and headers are defined. Define the concrete scope with maintainers before changing anything; done should include accessible contrast and clearer, convention-aligned controls across the affected themes.

Written by the indexing model from the issue text.