QT themes missing when using env QT_STYLE_OVERRIDE

hey guys, I am trying to theme a QT application (4kvideodownloader portable, on Cinnamon DE) via the env QT_STYLE_OVERRIDE=kvantum-dark method, however when I do it I get the error

QApplication: invalid style override 'kvantum-dark' passed, ignoring it.
    Available styles: Windows, Fusion

However when I run a different QT program (SVP), I can use env QT_STYLE_OVERRIDE=kvantum-dark just fine, and when I give an invalid theme it gives

QApplication: invalid style override 'INVALID' passed, ignoring it.
    Available styles: HighContrastInverse, HighContrast, Adwaita-HighContrastInverse, Adwaita-HighContrast, Adwaita-Dark, Adwaita, bb10dark, bb10bright, kvantum-dark, kvantum, cleanlooks, gtk2, cde, motif, plastique, qt5ct-style, Oxygen, Windows, Fusion

How do I allow for 4kvideodownloader and other QT apps to be able to see the system wide available themes?