I have Breeze Dark set up on my Kubuntu PC, and both VLC (from edge channel) and Falkon (candidate channel), as shown are installed as Snaps.
I also tested KCalc Snap, and even it responds to the system’s theme (though not as well as the one from Apt).
I installed the same Snaps applications on my laptop running on Ubuntu 19.10 with Unity, installed Kvantum to theme my Qt apps, but it doesn’t seem to work. Is there anything I should install?
Edit: attached image from my laptop with Unity
Edit 2: VLC (stable and candidate versions) sort of respects the system them.