This server is running Otter, the Online TableTop Environment Renderer.
Otter is Free Software, according to the GNU Affero General Public License, version 3 or (at your option) any later version. There is NO WARRANTY.
You can download source code for this running server version. That includes the source code for all the included game piece pictures (shape libraries), and general information including build and use instructions.
The shape libraries have a variety of licences. All those included with Otter are available under at least CC-BY-SA 3.0 or CC-BY-SA 4.0. Check the source in "view source" in your browser, or the browser Developer Tools (in Firefox, use Inspector); that will also tell you the library piece filename. If you want to edit them you will probably want to start with the actual source code, because the SVG pieces as sent by the server have generally been fed though an SVG distiller.
Further information about licensing etc. is available here, and of course also in the source code.