Excellent post and suggestion. I wish I saw that before I tried upgrading. I am also using the Fordson theme.
I my case I upgraded from 3.4.1+ to 3.5+.
Without going into details, the conclusion is that it should be recommended to switch to the Clean theme BEFORE doing the upgrade.
Not even the Boost theme was working properly.
Not sure if this should be handled by the Moodle upgrade script as Fordson is a plugin.
Maybe an alternative would be do download/pull the latest Fordson 3.5 and add it to Moodle 3.5 source before trying to logon.