GIMP running on .nix distributions that support the Gnome desktop environment can be used with custom themes. Following is a brief how-to which covers doing this on OpenIndiana oi_151a x86, with GIMP 2.6. The method should be very similar or identical for Linux distributions (e.g. Ubuntu).
First, let’s locate a sample GTK theme. I am using a rather nice theme called “Darkilouche” found here:
Simply download the compressed file in the above link, and extract the contents to disk.
Next, we need to locate the gtkrc file in the uncompressed theme folder. The sample theme we are using contains the following files and directories once uncompressed:
$ ls -al total 7 drwxr-xr-x 4 dave staff 5 2012-03-07 22:47 . drwxr-xr-x 3 dave staff 3 2012-03-07 22:47 .. drwxr-xr-x 6 dave staff 9 2007-02-28 00:22 .svn drwxr-xr-x 3 dave staff 4 2007-02-28 00:22 gtk-2.0 -rw-r--r-- 1 dave staff 240 2007-02-28 00:23 index.theme
The gtkrc file sits within the gtk-2.0 directory, and appears to be the only critical file that is needed.
Next, we copy the gtkrc file into the GIMP themes directory. In this example, I am using the global GIMP themes directory at /usr/share/gimp/2.0/themes. We simply create a directory named after the theme we are installing (in this case “Darkilouche”), and copy the gtkrc file into it.
Finally, we launch or restart GIMP and test that the theme is accessible at Edit -> Preferences -> Theme. The following screengrab illustrates that GIMP has found the theme, and, it is also the active theme: