[renewiltord]

You can use a userstyle on the webpage. It works. It's quite straightforward actually.

[MavisBacon]

I think most people are going to give up before they find that link in the footer, and regardless these themes still present accessibility issues, especially that static image background. I personally find even the "mostly monochrome" style very difficult to read through and I'm guessing I'm not the only one.

[bombcar]

They even offer ten themes and a chooser which is 9 themes more than 99% of websites.

[jrm4]

On this page, sure. On all pages, not so much.

[renewiltord]

No, explicitly on every page. It's a user agent level choice.