If a GTK application doesn't look proper, please open System Settings > Application Style and then, near the bottom, click on Configure GNOME/GTK Application Style .... In the window that opens, choose a suitable GTK2/GTK3 theme but preferably not Raleigh. You'll see an approximate preview of your choice as well.
KDE Plasma 5.18 has moved things about a bit.
If a GTK application doesn't look proper, please open
System Settings > Application Style
and then, near the bottom, click onConfigure GNOME/GTK Application Style ...
. In the window that opens, choose a suitable GTK2/GTK3 theme but preferably notRaleigh
. You'll see an approximate preview of your choice as well.