Hmmm... I understand your approach, on my system I chose gray color, so I did not pay attention to this aspect. For testing, I changed the color to something close to teal. But... "teal" will only be visible if there is
no color defined for
AlternateBase in plugin_utils.py.
But now it
is defined, plus it is the same color as
Base.
Therefore, however, I vote for the color R66 G66 B66, to preserve the "sigiliness" of the palette in dark mode.
Note that this leading color selected in the Windows settings is not
AlternateBase at all in light mode, so please consider this.
Nevertheless – I'd love to see the problem you mention, in which "dark theme will be stuck with a color that may not be optimal." Because, after all, we are only talking about Windows.