Scrollbar color follows OS mode, not Furo theme light/dark mode

[pradyunsg]

Discussed in #689

^{Originally posted by ShawnHymel August 10, 2023}
OS: macOS 13.4.1
Browser: Chrome 115.0.5790.170

The scroll bars on the page are always dark if my OS theme is set to "Dark." As you can see in this screenshot, even the official Furo documentation site has dark scrollbars. They should follow the theme settings and only revert to the OS mode if the "Auto" mode is selected on the site.

This is a bit tricky on Chromium browsers, since the scrollbar lives on html but all the state for the CSS theme is kept on the body tag instead of html.