Most of the WiTS source code is covered by a BSD-style license that allows for both personal and commercial use and redistribution with a few simple requirements in terms of maintaining copyrights and such. Please see the LICENSE file in the root of the source directory for details.
However, many of the images and icons used in the program have come from the Nuvola distribution which is covered under the GNU Lesser General Public License. If you are planning on redistributing WiTS (modified or not) without publishing source code, please make sure you are not in violation of this license. The exact terms of the license are in the nuvola subdirectory in the source repository. Alternatively, you can replace all the images and icons and rebuild the binaries.