Using gtk themes
From Fluxbox-wiki
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Where to get GTK themes
You can get GTK themes from for example FreshMeat's GTK section
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How do I install these themes?
You only need to untar those themes under ~/.themes, so first
mkdir ~/.themes
and then
cd ~/.themes tar -zxvf theme-name-here.tar.gz or tar -jxvf theme-name-here.tar.bz2
Make sure you check the readme for what engines the theme needs. You can most likely install them through your package manager, like
apt-get install gtk-engines-industrial or apt-get install gtk-engines-* or emerge gtk-engines-industrial
You should get the idea.
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And to use them?
Get gtk-theme-switch (note that there is also a version for GTK2) or gtk-chtheme. Then just fire 'em up and you should see a nice list of all styles. Just select the one you want to use and click Apply

