as a start you could look around in GIMPs themes folder .
Most of the work is editing a GTK theme configuration file, the gtkrc.
This is a text file. You'll need to read and understand it, but it's
no rocket science. There's also a GMOME tutorial on theming  and
lots of other themes in the web you can learn from, for instance on the
GNOME art site .
The icons are in GIMPs 'themes/Default/images' folder. It's also worth
knowing that the GNOME Design team worked on a suitable icon set .
To join your forces you should contact the GNOME visual design lead
Jakub Steiner (jimmac)  or our user interaction architect Peter
Sikking. You find at least jimmac on IRC in #gimp or #gnome-design.
Preparing your theme for deployment is - putting your files all
together in a folder like the already existing themes.
IMHO GIMP shipped with a modern, good-looking and unobtrusive dark
theme would be a nice thing, so your contribution is welcome
I think the gimp-developer mailing list would be a good place for
ongoing discussions on that.
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