Picard currently forces the use of the Fusion Qt style on all platforms except macOS and Haiku. It is possible to opt-in to the Fusion style on these platforms as well by setting the PICARD_FORCE_FUSION to a trueish value (1, true, yes, on).

It is not possible though to opt-out of using the Fusion style.

I suggest to allow setting PICARD_FORCE_FUSION explicitly to a falsy value to opt-out. Having PICARD_FORCE_FUSION not set at all keeps the platform specific default behavior.