I found this problem can occur with the following combination:
- system light theme
- Shotcut default Fusion Dark theme
- The environment variable
QT_QUICK_CONTROLS_CONFis set. Shotcut needs to set this on itself to set the palette for some components. However, it will not if it is already set (respect the user-defined env var).
Change one of these and the problem goes away. I have a KDE Neon (based on Ubuntu 24.04) system, and it does not define that env var.
Run echo $QT_QUICK_CONTROLS_CONF in a terminal to see if it affects you. The path might reveal a reason why it is set. If set, another workaround is
$(unset QT_QUICK_CONTROLS_CONF; shotcut)