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

Beginne damit, die in der Issue verlinkten Kontrastbeispiele der Splash-Page zu prüfen, und untersuche anschließend, wie die Pride-Themes sowie die selbst implementierten Buttons und Header definiert sind. Lege den konkreten Umfang mit den Maintainer:innen fest, bevor du etwas änderst; abgeschlossen ist die Aufgabe, wenn barrierefreier Kontrast und klarere, an Konventionen ausgerichtete Bedienelemente in allen betroffenen Themes vorhanden sind.

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 and the main (?) theme.

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!