I had a similar problem when upgrading from Moodle 3.6 to 3.8+ with Fordson, with the error
Fatal error: Class 'theme_boost\output\core\files_renderer' not found in /home/saheducation/public_html/moodle/theme/fordson/classes/output/core/files_renderer.php on line 38
I just renamed the file to files_rendererORG.php after Mary's post and it seems to have fixed the issue, fingers crossed.