You can easily install new GTK themes, just follow this steps:
First download the theme and check that it contain xfwm folder, for example you can get my favorite one Raleigh, old but gold.
Now you should move the themes and icons folder to specific paths.
- themes/Raleigh-Dark ->
$HOME/.local/share/themes - icons/Raleigh-Dark ->
$HOME/.local/share/icons
Enable the theme and also you can read more how to config gtk in ArchWiki.
$ gsettings set org.gnome.desktop.interface gtk-theme Raleigh-Dark
- Yaru Theme Most of the people not gonna like Raleigh, for them I suggest Yaru. It is a bloat one so it take some minutes to clone, also you can just get latest tag.
After cloning or extracting the compressed tag you should build and compile it:
$ mkdir build; cd build
$ meson setup . .. -D prefix=$HOME/.local -D buildtype=release
$ ninja install