Slim Scrollbar replace default scroll bars with two simple auto-hide bars. These bars are visible only when you scroll the page. So you obtain more space, useful especially for small screens.
I have no more time to develop this extension, so I don't know how many updates I will do in future. But you can try another actively developed scrollbar from Christoph142: Modern Scroll.
The extension can't automatically turn off the default scrollbars. To do so, open Preferences ▹ Advanced ▹ browsing, and deactivate "show scroll bars".
A well-know issue on oldest computers is performance degredation during scrolling. You can try to activate "solve slow scrolling" options, but this may result in the scrollbars flickering.
Thanks to Daniel (https://twitter.com/#!/Aeyoun) for the Norwegian bokmål translation and code support.
Thanks to Piotr Wójcik (http://my.opera.com/chocimir) for Polish translation.
Thanks to myonno for German translation.
Thanks to Güray Sun for Turkish translation.