Starting with OBS version 28 an internal QT style is enforced in OBS.
This style enforcement breaks the custom qt gnome platform theme official Fedora provides and causes OBS to not follow light/dark theming (notice the title bar):
We do not need or use these themes in Nobara, and they have been removed from the installation, however older installations may have them installed. To resolve the issue we can remove them. They are safe to remove:
sudo dnf remove qgnomeplatform-qt5
sudo dnf remove qgnomeplatform-qt6
After removing them and relaunching OBS it should resolve the issue (again, notice the title bar):