Here's how you can give GIMP a whole new look by installing the flat icon theme.
1. Open a terminal window.
2. Type in the following commands then hit Enter after each.
cd && wget -O flat-themes.tar.gz http://goo.gl/ZIEGnF
tar xzvf flat-themes.tar.gz && cd flat-themes
sudo mv * /usr/share/gimp/2.*/themes/ && cd .. && rm -r flat*
3. After the install, go to Edit then Preferences then Theme to apply your new theme.
Many people don't like GIMP default icon theme/look and want to change it with something else, so here comes 8 variants which includes: dark, light, darker and also variants with small fonts. Using these themes you can make Gimp experience better and eye catching.More at Noobs Lab
Additional resources for GIMP