Hello!
I have a client who wants me to transform GitHub theme to GRAV theme.
I’m experienced Web designer, but my main field is WP.
But, I’m always open to learn new things.
That’s the reason I accepted that project.
But, honestly, I don’t know how to do it?
Any help, please?
Thanks,
Marin
welcome to Grav 
for theme development, you should first read the Tutorial .
then you will see it is quite easy to get a solid starting point.
and, yes, you will also notice many things that are very similar to e.g. WP Theme development - e.g. css stuff.
and, of course, others you might not (yet) be used to - such as templates.
in Grav, these make heavy use of the twig templateing language which makes this far more logical than the html/php mix in WP 
.
good luck !