Scrollbar styling

Themeing of scrollbars through standardized `scrollbar-color`/`scrollbar-width` and `::-webkit-scrollbar` pseudo-elements so they match a site's palette without harming usability.

grounded in: commits 0200ef2 'Theme the scrollbar corner to the phosphor palette' and 6ad021d 'Brighten the scrollbar thumb while it's being dragged'