Is possible to change scrollbars' style for all browsers? If it is, how?
It is possible in Internet Explorer using a number of non-standard scrollbar-*
CSS properties. See this page for a handy generator tool.
Other than that, it is possible only using custom JavaScript-Powered scrollbar solutions. The jScrollbarPane jQuery plugin looks very nice and easy to install. Here is an examples page.
Nope. IE allows you to set colours for some constituent parts of the scrollbar. Opera allows a few but not all of those styles.
Scrollbar colour styling is of increasing irrelevance as UIs move towards complex image-based scrollbar theming. In IE, setting any of the colours reverts the rendering back to a Windows 2000-style simple-3D scrollbar instead of any swishy user theme. Windows Vista/7 (Aero) users probably won't thank you for that.
You can of course make your own ersatz-scrollbars out of <div>
s and style them how you like. But the result almost always behaves worse than real scrollbars, since you're trying to replicate a complex UI element whose expected behaviour is different for each OS. You can spend a lot of time reproducing paging behaviour, keyboard up/down and the mouse wheel, but it'll never quite feel as smooth a real OS scrollbar.
You can style scrollbars for all browsers with a little bit of Javascript. But at present time there is no way to style them using just CSS alone as a cross-browser solution.