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

先查看 issue 中链接的 splash page 对比度示例，然后检查 Pride 主题以及手工实现的按钮和页眉是如何定义的。在进行任何更改之前，先与维护者确定具体范围；完成标准应包括在所有受影响的主题中实现可访问的对比度，以及更清晰、符合约定的控件。

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!