As I see there is a version for Moodle 3.5 (https://github.com/dbnschools/moodle-theme_fordson/tree/moodle35) and the master is on development for Moodle 3.6.
So, before you upgrade the Moodle database you must verify that you checked out the right branch "moodle35" inside the theme/fordson folder. On my local Moodle 3.5 env. it's running without problems.
If you want to split up the upgrade of the Moodle core and theme, I suggest you to change the theme to "boost" and do the Moodle upgrade and afterwards you upgrade only the theme.
Additionally here's the official support forum https://moodle.org/mod/forum/discuss.php?d=380975