@overdress188 ,
1st) install qt6ct app using your linux version package manager.

2nd) are you using bash shell? if you are, in your home folder there is .bash_profile and .bashrc files in your home directory.

open .bash_profile (or .bashrc if you want the environment variable set there) and add the following line:

export QT_QPA_PLATFORMTHEME=qt6ct

3rd) open the qt6ct program you installed from your package manager, its a gui prgram not commandline.

On the Appearance tab (first tab) you will see a setting for color scheme: set it to 'darker' and click apply.

Logout, log back in and now the qt6ct setting you just changed will enable 'darker' mode in Strawberry.