if you are running this inside distrobox. make sure you also install gnome-icon-theme or whatever the package is called on the distro you used for distrobox. this will add the fallback icons that all gtk applications expect to be on the system.
loading the darktheme you can install the breeze gtk theme (as long as you are still using the default themes from bazzite kde) and it should auto pick that up (assuming the theme is gtk2 compatible)
as i can see the text fine in the image but you are missing the icons.
also keep in mind that if the application is gtk2 based, there is a high chance that dark theme will not work well on it (dark themes used to have lots of issues with gtk2)