With Qt6 alpha on KDE and the "kde" platform theme, Qt selects a non-optimal font style:

The screenshot shows a qdbusviewer launched with QT_QPA_PLATFORMTHEME=kde under KDE 5.20 beta. The font is Roboto, and the style seems to be Extralight, which is super hard to read.

Relevant font settings in ~/.config/kdeglobals:

fixed=Monospace,10,-1,5,50,0,0,0,0,0
font=Roboto,10,-1,5,50,0,0,0,0,0
menuFont=Roboto,10,-1,5,50,0,0,0,0,0
smallestReadableFont=Roboto,8,-1,5,50,0,0,0,0,0
toolBarFont=Roboto,10,-1,5,50,0,0,0,0,0

Note that the specified fonts do not specify any style.