I haven't found any settings to adjust this.
The active virtual tab is white white. The non-active are..uuuhh..white - perhaps a slight graying but that's because I'm forced to describe it :-) The difference just doesn't pop out.
I've tilted the screen and adjusted brightness to see if it's perspective, but that isn't it. Is there a setting, or can @kovidgoyal create one so that active virtual tabs can be highlighted in a better manner?
I tend to think I'm in one library while actually in another.
It would be very helpful !