Improve scroll bars and support dark mode

[choldgraf]

Context

Currently, our scroll bars use the default system style. This means that they look a bit "clunky" and also don't support styling via dark mode. For example:

Proposal

We should port the book theme scrollbar style to this theme, and add a set of dark mode variables as well. This would make the scroll bar more aesthetically pleasing and also improve the UI in dark mode.

Here's an example of the scrollbar style:

How to implement

The scrollbar style is defined here as a mixin, and could be adapted pretty easily:

https://github.com/executablebooks/sphinx-book-theme/blob/940d562fad0c8e9d4056b575c68d972ac0f9b2f4/src/sphinx_book_theme/assets/styles/abstracts/_mixins.scss#L6-L21

[damianavila]

We should port the book theme scrollbar style to this theme, and add a set of dark mode variables as well. This would make the scroll bar more aesthetically pleasing and also improve the UI in dark mode.

+1 on the proposal!

[grahamgower]

Default scrollbars styles are the best. With this new scrollbar style, its very hard to see the scrollbar, and very hard to click/grab it. It's just too small, and making matters worse it disappears when the cursor is over it. I firmly believe that functionality is more important the looks here.

There are some useful guidelines here: https://www.nngroup.com/articles/scrolling-and-scrollbars/

Here's how the current scrollbar looks in chrome on my system.