User can choose between 'GiD Black Vectorial', 'GiD White Vectorial', 'GiD Classic', or other themes, which change drastically the GUI appearance. User can also choose between some icon and text sizes in each theme.
These options can be changed in GiD Theme option inside Utilities->Preferences->Graphical->Appearance tab.
Note: vector-themes have images in SVG format and can be scaled without loss of quality, on the other hand the Classic theme has raster-images (PNG, GIF,...)