Ah, that's because GTK4 and libadwaita severely restricted custom theming. They may be ways around this but I have no idea because I use just adw-gtk3 as my theme so all my apps look uniform on Gnome.
So I guess you'll either search for a way to hack this or just stick to GTK3 apps if you don't want the stock GTK4 look.
It’s not nearly as supported. There are workarounds where you can create a ~/.gtk4 directory and modify CSS yourself, or you can use a program like Gradience to modify the color scheme in an accessible way. Gradience also has community color palettes so if you’re using a popular theme it could just be a matter of loading the preset