I just tested the Read Only option of with Delphi 10.3 professional. It works fine with GExperts installed, so it probably isn't the culprit.
Also, disabling GExperts from the experts manager while it is acive in the IDE works fine for me (after restarting the IDE, GExperts is gone).
Enabling it via the stand alone Experts manager brought it back.
So everything seems to work as expected.
I do use GExperts every day.
I don't use Delphi 10.3. I only have it installed to compile and test GExperts. And every time I have to use it I curse it.
I don't use the dark theme and I have got lots of more interesting things to do than implementing theming in GExperts. I already had enough trouble to keep GExperts functional when Embarcadero changed the light mode to also be a theme.
So, unless somebody submits a patch for this and fixes all the bugs it will most likely cause, theming in GExperts won't happen.