Breeze for iced and Cosmic for Qt. That’s the elephant in the room, I’d say. Another solution would be to attempt a common theming system for Qt and iced. Both solutions are particularly high effort, but it’s easily what’s going to have the most noticeable impact.

I assume the Cosmic people already know this, but it’s also important to make an effort to follow the usual cross-desktop standards, like the icon-theme-spec, icon-naming-spec, sound-theme-spec, org.freedesktop.appearance accent-color, org.freedesktop.appearance color-scheme or org.freedesktop.portal.FileChooser.

It would also be nice to support a few unofficial standards KDE uses, like appmenus.