I’ve had this problem before with virtualbox showing white, washed-out menu buttons/letters, and was able to fix it (on another distro), by adding “-style fusion %U” to the command.
Tonight I downloaded qGo, a client for gnugo, and got the same problem. The above solution did not help. I’m using Xfce and Arc-Dark as theme.
There isn’t a “silver bullet” here - you’ll have to decide whether you want to configure the same theme for both GTK and Qt, or use something like qt5ct.
Configuring a common theme might be something to add to the installer image for Xfce, at least.