Low contrast color

Hello. I have a my color scheme, good with some apps, such as Dolphin, but not with other ones, like Waterfox or Libreoffice: too low contrast between the white text and the light olive background.
You can see it in my attachment.
What should I change to fix this problem?

Thank you!

EDIT

I noticed that it’s a gtk theme problem.
Therefore my question now is: how to customize a gtk theme?

my system
Operating System: KDE neon 5.27
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.113.0
Qt Version: 5.15.11
Kernel Version: 6.2.0-39-generic (64-bit)
Graphics Platform: X11
Processors: 4 × AMD Ryzen 3 3200G with Radeon Vega Graphics
Memory: 5,7 GiB of RAM
Graphics Processor: AMD Radeon Vega 8 Graphics

You can change the GTK theme in the Application Style settings, and clicking the button “Configure GNOME/GTK Application Style”

Thank you, but I knew that. That way you can choose a predefined style. However I was searching a way to customize a gtk theme.

GTK themes can’t really be customized easily by the user. It seems like the issue in question is a bug in the theme that must be fixed by its developer (maybe that person is you if you developed the GTK theme).