Kvantum VS lxqt-themes

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.

For qt you can take a look at lxqt themes and Kvantum for instance.