Interesting. I use the hidden HTML only version of gmail, so I won't see what you are talking about.
https://addons.opera.com/en/extensions/details/modern-scroll/?display=en will customize scrollbars. Initially, this will make sites look worse, but if you go to the extensions folder and open the extensions folder and then open the most recently created folder then go into the css folder and edit the ext.css file you can edit the look of the scrollbar.
C:\Users\Your Name\AppData\Roaming\Opera Software\Opera Stable
The extension looks simple enough that I am tempted to take a stab at creating an extension myself.