Hi Jason,
I could not find the exact commit, but it seems that the introduction of StructureController caused a problem.
To reproduce, display the structure of a database, tick the checkbox for one of the tables, and in the drop-down "With selected" menu, try to optimize this table.
Notice in ./libraries/mult_submits.inc.php#254 Undefined variable: pmaThemeImage
Backtrace
./libraries/controllers/StructureController.class.php#139: include(./libraries/mult_submits.inc.php) ./db_structure.php#45: PMA\Controllers\StructureController->indexAction()