There is one
gtk2 theme that I use for
gimp 2.6, although it should work for
gimp 2.8, and that is
Darkilouche. (It will work until
gimp switches to
gtk3 around the time of
gimp 3.) It may not be quite as dark as the one you have in mind, but it's good and you may be able to modify it further. It was created by a while ago here and the version you want is 0.2.3, available here as well.
Needless to say, it is easy to install. Just unpack the zip and create a new folder called
Darkilouche and place in it the contents of the zip (folders labelled
.svn and a file named
Darkilouche. Then move the
Darkilouche folder to
.themes in your home folder. Then, to test it with
gimp-2.6 for your version), run:
This command simply specifies a particular theme (
gtkrc) to use with a
gtk application, it will NOT affect your general Ubuntu theme or interfere with any other programs.
Then you can create a launcher to run
gimp like this in the future.