KDE 5 system window buttons black with no text

Debian Sid, with multiple desktop environments installed. After update, in KDE Plasma 5, all system buttons on all windows are black, with probably black text, just black squares or rectangles. Everything in the panel - icons, app window indicators, everything - is that way, and it's the same in all settings windows. Application windows have the black squares for the minimize, maximize, and close buttons. I've googled and tried changing everything I can find to no avail. I purged KDE and reinstalled, but the issue remains. It would appear to be somewhere in the QT5 settings but I have found nothing. Other desktop environments are completely normal, including Xfce.LXDE, and Openbox. I've spent all day on this, and I'm out of ideas. Any pointers would be appreciated.