Here there are some themes for your GNUstep Apps. Mainly these themes are for people that use desktops like Gnome, KDE, XFCE,... Just untar the package and copy it on ~/GNUstep/Library/Themes at your home directory. You can set the theme with SystemPreferences.app.
GNUstep SVN Themes
GnomeTheme is a plugin to make a GNUstep theme based on the active Gnome theme.
WinUXTheme is the theme for Windows. You can active this on installation or with SystemPreferences.app.
You find more at GAP.