These assets were added by mistake a few commits ago as I was not aware that they had changed their Creative Commons license in 2018 to a NoDerivatives license. The assets now retained were either released under the old ShareAlike license, or is freely available graphics such as system logos.
Added dummy SVG files to all missing graphics to avoid error messages when loading such systems. Also standardized on a single theme.xml file layout for all systems. A separate theme.xml file is still needed for each system, but now they're at least identical.
