Reading the upstream code, it looks like setStyleSheet(QString()) will unset any custom style sheet. This seems like a cleaner solution because setting a stylesheet at all interferes with changing the widget colorscheme from a dark to a light scheme or vice versa.
- open konsole
- open systemsettings and change the colorscheme to a dark one, if you're already using a dark one, change to a light one
- widget colors are changed as expected, except for konsole tabs, which look odd
Opening a new konsole window seems to clear the issue and the tabs look normal again.