Finally got around to trying to fix this again and did so successfully.
The fix? Copy my downloaded/custom themes from .themes
, .icons
, and .config/kvantum
to the root folder
. That’s it! It was really that easy the whole time.
Now, dolphin and dunst, and even qt5ct are rendering fonts and other UI elements at the expected size.
Note: This is an issue I probably would not have if my themes were installed from an official repo or the AUR.