You should'nt rly bother with theme, ofc if you really want to do it.
It will be non-stop time sink, time you could use to develop something in C3, get better at it or get better at something else.
Past simple color change, you would need to:
*Make selection, hover, focus colors, backgrounds and borders. Also selection+focus and selection + hover. Then also consider if something behaind is already selected/hovered/focused and account for that
*Need to change all coding colors in multiple places for all types of files.
*Colors for new mesh, sprite and all other object types. Also when they are selected and modified etc.
*Custom coloring for all the main dialogs and all other small dialogs.
*Everything you do, you also have to make sure they look good, if you make them very big or small.
*And bunch more cases, if you like to change something past color.
You would also have to consider, if something is missing, then you have to get it from Default theme or add it.
My aim was to get better with CSS, while editing something big as C3 editor and change thing to how I like them to be. So I did'nt really care that it was huge time sink.
Anyway, good luck with it.