Issue #3364885 by andy-blum, mglaman, alexpott, mherchel, lauriii, smustgrave,...

Issue #3364885 by andy-blum, mglaman, alexpott, mherchel, lauriii, smustgrave, isholgueras, Dave Reid, kostyashupenko, matthieuscarset, camoa, ctrlADel, fjgarlin, jwilson3: Allow themes to use a starterkit.yml file so it is easier to use the theme generator

(cherry picked from commit 6c2ed729)