No problem for me.
As soon as I make a change in the settings, it becomes a 'Custom' scheme and the 'Store As' button becomes active.
Are you in a Teamwork file by any chance?
Windows or Mac?
If Mac, is it Ventura OS?
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10