I am moving this back to Installing and Upgrade Help Forum
as this is not a Theme problem.
However, before I move this discussion can you please tell me which theme you where using when you did the Up-Grade?
This is important because if it was the Essential theme, then there may be problems connected to that theme which prevents Moodle from finding the Default theme. This is just a wild guess as Essential is a unique theme that is, in effect, self contained.
It maybe that the version of Essential you have is NOT COMPATIBLE with Moodle 3.3 and therefore may be the cause of the problems you are faced with.
Thanks
Mary