Other desktop environments or compositors will need to use Qt configuration tools called qt5ct and qt6ct, by passing them as an environment variable as follows:
QT_QPA_PLATFORMTHEME=qt5ct

Like any other UI library, Qt theming can also be split into widget styles, window decorations, colour schemes, icon themes, cursor themes and fonts.

Out of these, cursor themes and icon themes are fairly generalizable to other libraries too. But widget styles, decorations and colour schemes will be tied to the library.

Instead of using qt5ct, one can also use a theme engine named Kvantum.

Using Kvantum

Install Kvantum for your distribution. It will act as a theme to Qt apps.

Set Kvantum as the default theme using the environment variable:
QT_STYLE_OVERRIDE=Kvantum

Then use Kvantum Manager to install your desired Kvantum theme.

Applying themes in Flatpak

Setting Style

Firstly, install the required KStyle: `flatpak install org.kde.KStyle.
(I noticed that with Kvantum, I had to install all versions to fit all apps)

In Flatseal, set:
Environment Variable: QT_STYLE_OVERRIDE=theme

For Kvantum, add filesystem access to Kvantum install folder:
Filesystem Access: xdg-config/Kvantum:ro

Setting qt5ct

Set environment variable: QT_QPA_PLATFORMTHEME=qt5ct (Haven't tried this though)

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