@redbatman There's should be a tutorial in the docs, also looking at the themes.md document (which was linked in the tutorial last time I've looked at it) can be helpful.
And yeah, getting a theme that looks close to what you want and change things is a good way to start. Then you can look at even more themes to understand how to change positions and whatnot.
It might take a while to understand everything but on the other side, there's not that much to learn.
If you have a Windows PC, I also suggest to get the Windows build of EmulationStation, with that you can edit your files and immediately check the changes.