It is relatively easy to create a theme yourself - it boils down to creating a .json-file and sideloading this into MapComplete. The entire process is explained here

Once it is finished, the next step is to send it back to me (preferably via a Pull Request) to get it into the “official” mapComplete. This offers some benefits, mostly that it can be discovered by others, that it’ll be translated and that documentation can be automatically generated (for example: tagInfo shows which projects use a certain tag, those are auto-generated)

I’m currently building an easy-to-use ‘MapComplete-Studio’, so hopefully, it’ll be even easier in a few weeks time!