Darken scroll bars in dark mode

[leifwalsh]

In #1337 the custom scroll bars were removed for accessibility, but now they look starkly bright again in dark mode, and really cut the primary navbar away from the main content.

https://stackoverflow.com/a/66097469 shows a happy medium: setting color-scheme: dark when the dark theme is selected will fix the scroll bar coloring (and presumably other browser-default things, like unstyled input elements maybe).

I don't know scss enough to propose a diff but hopefully this is easy for someone?