The only work-around I've found (Monterey 12.0.1) is to go into your system preferences and force light mode when you're working on BS3. In the BS3 prefs, select "Look and Feel" and select "Grey" under color scheme. At that point (if you have forced light mode in your system prefs), it will resemble the classic BeerSmith look. However, you have to change your system prefs back and forth if you want dark mode in other apps.