qt6ct VS Kvantum

kvantum can use arc theme and apply it for qt's app with qt5ct and qt6ct

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

For instance, if I open QBittorrent right now (which has been recently ported to Qt 6), it's clearly using the Fusion theme because of that.

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.

I'm using Kvantum engine with my custom Andromeda theme

I think Kvantum has support for theming both Qt6 and Qt5 apps now though so that might be worth trying if it bothers you that much.

The ones I know of are: Aurorae (built-in one), Kvantum (https://github.com/tsujan/Kvantum/tree/master/Kvantum) and Lightly (just a theme https://github.com/Luwx/Lightly). store.kde.org is down (at least I can't access it at the moment), so I've shared github links.

What about : https://github.com/tsujan/Kvantum ?

Make sure Kvantum is selected in the application style section of your look and feel settings. Read through the usage instructions on the Kvantum GitHub page.

Perhaps you can submit an issue

As for Kvantum, you'll need to either install it from your distro's repos if you're lucky, or if not you'll need to compile it yourself, you can check how to do that here.

arc-theme with kvantum look petty good imo.

It's written in QT, so you need a QT theme. There's the KvLibadwaita style for the Kvantum theme engine.