Most if not all major browsers let you style scrollbars, but the problem is that styling for built-in widgets is not standardized, i.e. it relies on vendor prefixes and worse, not all (sub) elements can be styled at all depending on browser. E.g. <select> has poor support for styling.

Yep - I remember on vBulletin forums, the native vertical scrollbar often got the same vibrant colouring as the forum's theme.