I had some gtk apps that did not want to theme correctly in KDE Plasma, so this is how I resolved it:
$ sudo pacman -S xsettingsd gtk-engines matcha-gtk-theme
Then I setup the xsettingsd configuration:
$ mkdir ~/.config/xsettingsd
$ nano ~/.config/.xsettingsd/xsettings.conf`
Gtk/EnableAnimations 1 Gtk/DecorationLayout "icon:minimize,maximize,close" Gtk/PrimaryButtonWarpsSlider 0 Gtk/ToolbarStyle 3 Gtk/MenuImages 1 Gtk/ButtonImages 1 Gtk/CursorThemeSize 24 Gtk/CursorThemeName "breeze_cursors" Net/IconThemeName "breath2-dark" Net/ThemeName "Matcha-dark-sea" Gtk/FontName "Noto Sans, 10"
Finally I reinstalled kde-gtk-config to make sure it picks up the optional dependency for xsettingsd:
$ sudo -S kde-gtk-config
I then rebooted, but I suspect a logout would work too. Now apps like pamac-manager theme correctly.