[code] Style scrollbars using the theme colors

[pomber]

Maybe we can use something like this postcss plugin: https://github.com/pascalduez/postcss-scrollbar, and set some CSS custom properties using the theme colors.

[pomber]

Docusaurus uses thin scrollbars

maybe we can add that too

[pomber]

After some tests, using the theme colors can be done, but it overrides the MacOS behavior of auto-hiding the scrollbars, so it would be a worse experience for people that prefer that behavior.

What we should do is at least set the color-scheme property that matches the theme.

BTW, Firefox supports the standardized scrollbar CSS properties, and those properties don't override the MacOS behavior.