I try to change the color of the current line in the debugger, since the contrast is very bad, you can barely read it.
I searched all options under Fonts & Colors but I could not find it.
It even persists after applying a new theme.
To change the background color of the current line within the debugger:
Select the new Background color, which is yellow in the example below, and click Apply.
In my case I tested this by debugging a servlet. Simply redeploying the application, or restarting the application server, did not trigger the background color change. Instead I needed to close and reopen the edit window of the source code that was being debugged for it to become effective. You could just restart NetBeans instead, but that seems like overkill.