I think the tough answer, Desprez, is that you can't do a whole lot on the mac. This has been the holy grail for many a folk for a long time now, with only limited success. The only RSB converter out there only converts RS RSB files (the GR RSB files are actually a completely different format, aside from the extension).
Changing fog distance, color etc can be done pretty easily with a text editor and tweaking the .env file, like mainman said. Weather can be changed as well, by changing what sky texture is loaded (i think, not an expert on that). Dr. NO has done alot with weather modding, so he'd be a good guy to ask. (you're welcome Doc

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As for changing the ground textures and lighting, the applications are just not there for Mac. You'd need a PC for that. The PC version of GhR comes with a map editor, which was so gratefully left out of the Mac version, the bastards!