GitHub theme transform to GRAV theme?

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 !