In previous versions there used to be an option called XDefaultApplicationStyle in the Inspector. This option in no longer present in Mathematica 12. How can one change the widget style from (I think) "fusion" to, say, "Breeze"?
If you can still do this on Linux, I think you will need to set some Qt option because, if I read Stephen Wolfram's blog post introducing V12 right, Linux no longer uses XWindow. In his blog, Stephen Wolfram wrote:
There’s also a platform update on Linux, where now the notebook interface fully supports Qt 5, which allows all rendering operations to take place “headlessly”, without any X server — greatly streamlining deployment of the Wolfram Engine in the cloud.