The Qt Creator's theme is applied to the form in the form editor, unconditionally

If I use the dark theme in Qt Creator, the form I'm editing in the form editor (or the widget designer? don't know the official name) will also become dark even if I didn't apply any qss to it. What's worse, if I want to preview it, the preview window is also dark. It's a critical issue because I can't see the widget clearly at all if everything is dark.