The default combobox background color is white (QPalette::Base / Kirigami.Theme.View),
preview comboboxes were gray (QPalette::Window / Kirigami.Theme.Window)
Anything left to be done here? I guess this should land on the Plasma/5.13 branch, since it's still broken there (I almost wrote a patch…).
Patch LGTM, but there's still an issue (but not sure if intentional or not): The hover effect does not apply to the preview image, it stays white instead of getting a blue background.
Hmm, this doesn't look so great all on its own:
I think the graphical representations of the various anti-aliasing styles would also need to change their background colors.
Also the patch needs to be rebased on current master.