The defaults on Linux are horrible and very low contrast.
It is VERY difficult to locate the scroll handle widget.
Notice the difference on other applications in the image below.
Even non-native ones that don't take window manager theme hints use a better contrast.
This needs to be fixed. Even the rather infantile esesschema does a better job.
BONUS POINTS if you allow the system theme to be applied, but not needed if the contrast is better. 
