I don't know what GUI tools still exist for
manipulating
GTK2 themes in non-GTK environments—I remember it as being a
painful experience that drove me to the command line within an hour.
There is LXAppearance, which has both GTK2 and GTK3 versions. Also, if
gtk2-tqt-engine is installed, the GTK theme can be set via TCC.
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Mavridis Philippe