Using Fedora 27 and Guitarix: guitarix-0.35.2-6.fc27.x86_64
I still have the problem : light gray button with white font ... unreadable.
I tried the sed command, but it did not help (cannot see any difference.)
The workaround was to install Tweaks (Gnome Tweaks) and change the Application theme from "Adwaita (default)" to "Adwaita-dark" in Appearance.
Guitarix UI on Fedora: Difficult to see/read
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