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

先檢視 issue 中連結的 splash page 對比度範例，接著檢查 Pride 主題以及手工實作的按鈕和頁首是如何定義的。在進行任何變更之前，先與維護者確定具體範圍；完成標準應包括在所有受影響的主題中實現可存取的對比度，以及更清楚、符合慣例的控制項。

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!