SUMMARY With several different widget styles (e.g. Plastique, Oxygen (one with the highest contrasts), Breeze), the tab bar has too little contrast to easily see which tab is active. It would be nice to have an option to increase the contrast, e.g. by using a dark background color with bright text for the active tab when using light color themes for KDE widgets and vice versa.
This isn't likely what you wanted but it can be done in the Konsole settings, you can see a .css file that should allow what you want. I'm open to other suggestions on how to do this. https://docs.kde.org/stable5/en/konsole/konsole/tabbarstylsheet.html
A workaround is better than nothing. Can you reproduce or should I add a screenshot how it looks here? I have on the tab bar with Plastique theme: - 1-3 px lightgrey border above inactive tabs and the active tab is connected to the top border of the other tabs - Inactive tabs have a gradient with two light greys, the active tab has one solid lightgrey - The active tab is slightly higher overlapping the area below While this is visible when looking at the tab bar directly, it is hard to intuitively notice when looking at the terminal contents.