That might be because you lost your qt4 theming config when you upgraded your DE to qt5, so qt4 has fallen back to defaults.
On ArchLinux you can run `qtconfig-qt4` to configure the look and feel of qt4 applications.
On ArchLinux you can run `qtconfig-qt4` to configure the look and feel of qt4 applications.
I always was a great fan of both GTK2 (for the UI) and Qt4 (for the API). GTK3 made me switch to Qt4 for development. Qt5 brings some improvement, but as a user I absolutely hate QML.
Running qtconfig-qt4 and redefining the config I wanted did fix the "ugliness" of qt4 applications after I upgraded to KDE5 (or whenever it was KDE switched qt versions)