My 'dark' theme opened up today as a light grey.
I opened up preferences theme and it was still on dark so I tried the next one down (default) and it went to a dark grey

EDIT: I closed gimp and reopened then opened up the preferences dialogue. Put theme back to dark and all is ok now.

Sometimes it seems to depend on the "splash" image you use. With my licorice theme sometimes i have to start several times as if it's picky on which splashes it(my theme) likes. I know this is the case because i removed all but 3 splashes it never has a problem with and it opened every time on first start. I replaced all the splash images and it started getting buggy again.So i decided to use a default dark theme for start and after it loads, edit>pref>themes to get my "metallic blue licorice" theme back. After i am done with GIMP i reset the theme to dark default and close GIMP. No problems at all now.
On another note if you use only the default splash (no splashes folder), then it is probably the theme itself. Is it one you added to the themes folder? Or did it come stock with your GIMP download/install?

It is one that came with my Gimp Rod. I dont often install others.

GIMP-2.10 comes with quite a few themes. I am not sure about Parthas MAC version though. I believe his is shipped with a dark theme as default.