The only branding options currently available in GIM are on the Administration -> Preferences -> Look and Feel tab.
At this time the only modifiable options are the sidebar banner (and collapsed image) and the application text. There is no per-portal modifications you can make, as these are globally shared between admin, provisioner and self-service provisioner pages.
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If you're using NAC (Access Control Engines), you can get more granular with the self-registration configuration on the NAC portals in XMC, with color palettes and header/footers, etc.