Active Tabs of Partstacks are hard to spot

[laeubi]

Currently all tabs in a partstack are just gray with a small underline (in gray) on Linux/GTK

As one can see, for Editor tabs the current selected tab has a white background:

https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/discussions/2812

I find this really hard and one has to always carefully look what of these tabs is actually active