I know this has been discussed and rejected by the inner circle, but I want to open the discussion again, as the current themes have a lot to be desired. The main compelling reason I saw for not allowing QT native themes is that apparantly MuseScore is using a few custom controls which would not be themable using QT mechanisms, which would undoubtedly lead to odd behavior.
I would like to suggest that still MuseScore be allowed to support QT native theming, in some kind of backdoor mode, where the user knows they are crossing into unsupported territory by so doing.
The main advantage of this is that there are people out there who love to make wonderful themes. Just look at all the cool third party themes that have been created for Logic Pro and other DAW's by such people. As the theming ideas come in, the Musescore devs can choose to update the two built in themes to use some of those ideas to make MuseScore more visually appealing to work with.
My main suggestion would be to use more 3d effects or color effects to pop out certain UI elements. there is an art to doing this and perhaps most of the MuseScore devs are not really that good in that particular area. That's understandable, but the people that are good at that sort of thing have no scratchpad to try out some theming ideas, so please see my first paragraph for the request to allow QT native theming in some backdoor fashion for those that wish to mess with it. Why not?
I totally get the desire to keep the code under control, but being so "closed" stifles creativity, innovation and contribution.