I think it is not really a feature request but a bug report, as assuming the background should be white is plan wrong. It was good as a proof of concept, but not in a product.
Using the Toolbar customisation in a dark theme is impossible without that fix. The buttons are invisible.
Since the buttons displayed are the same as on the toolbar, it would be appropriate to use the same background colour from the toolbar or add an additional Toolbar::customisationBackgroundColourId, if you are worried about breaking changes.
A personal note: this is the only bug we have to patch in our CI… please help us to get rid of this necessity.