@ww2_1943 As of now, the only reason I'm here is to serve. I'm glad to hear that my project has made a positive difference in your life, if at least only a minuscule one in the grand scheme of things.
@minWi You may create an additional user account using any name of your choice, and then delete the default 'Mac' account promptly after. If memory serves, this should not affect any of the preferences or optimizations built into Sorbet Leopard by default.
The system theme itself shouldn't present any noticeable difference between it and the other Mountain Lion or Snow Leopard themes. The only performance penalty to my knowledge is in using the bundled High Sierra wallpapers, and since they're all a naturally higher resolution than the others, they therefore consume more real estate on the GPU's VRAM banks whenever displayed.
So if the system had an especially finite amount of VRAM to work with, from a performance perspective, it would be more desirable to either use solid colors exclusively, or to only use the optional Mountain Lion / default Snow Leopard themes, which the wallpapers of are all successively lower in resolution in comparison to the High Sierra collection bundled alongside LeopardRebirth (the base High Sierra system theme).
I hope that was helpful.