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Check if you have qt6ct installed.

In console: qt6ct

If not (most likely, Mint does not have it preinstalled):
sudo apt-get install qt6ct

Now you need to create an environment variable for the system to manage qt application themes with qt6ct. In console:
export QT_QPA_PLATFORMTHEME=qt6ct

That's it.

It will work for you, because I also have Ubuntu "Noble" base, and I have confirmed that qt6ct is available.

However, there are still distributions that do not have qt6ct yet, they only have qt5ct available. So although it is a solution, it is not a universal solution.