That's because there's no Breeze theme for Qt 6, so it's using the builtin Fusion theme which is literal cow dung. If you port your theme to Qt 6, the app should look identical.
I do not use a theme. I think you have it the wrong way around. What's missing here is the Qt6 equivalent of KDE System Settings → Appearance, or configuration tool <http://qt5ct.sf.net>, and it's not my task as the end user to provide these, but the toolkit and desktop environment developers'.
You always use a theme with Qt or Gtk. On Plasma the default Qt and Gtk theme is Breeze, which has not been ported to Qt6 yet. ATM the only Qt Widgets-compatible themes I have on my system are "Fusion" and "Windows" (as in "Windows 95"), both of which look absolutely horrible. Fusion is the default, and it's what you currently see when you open a Qt 6 Widgets app (for instance, QBittorrent right now).
By the way, qt6ct exists <https://github.com/trialuser02/qt6ct>, and it's even in Arch's repositories right now. The main issue is that it's useless, because there are simply no Qt6 themes out there yet and KDE still does not support Qt6 so you have to force qt6ct manually.
