TuxGuitar supports custom skins. The skins modify toolbar icons and dialog buttons.
The default skin for TuxGuitar is Oxygen, but TuxGuitar comes with other great skins. You can change the current skin in the Tools → Settings dialog, choosing the Skins tab. A preview of each skin is available.
To add a new skin, create a new folder in $TUXGUITAR_DIR\share\skins\ and copy the needed files into that folder. After that the skin should be available in the dialog.
Skins are contributed by TuxGuitar users, so if you are a talented designer, feel free to make your own skin or modify an existing one!