Ian No, you don't need to begin from scratch 😃. If I understand correcty:
- You had a working MoodleBox based on 3.8.0 image (with Moodle 3.9.x)
- You've installed Fordson theme on it, and it worked
- You updated the Moodle instance of the MoodleBox to 3.10.0, following the documentation
- Your theme breaks after this
If this is your situation (sorry, I'd to guess), then the problem is in the theme, and you'll have to update Fordson to its last version (the one that fixes its issues, provided it's released). Good luck.
PS. IMHO, this is a pure Moodle question, not specifically related to MoodleBox.
PPS. Before any Moodle update, you should check that all your third party plugins are compatible with the updated version of Moodle, otherwise you'll often break your Moodle installation.