Link matching locator "'Add slots'" not found.

Due to some theme changes almost all scenarios for mod_scheduler fail:

{{Scenario: A teacher edits a single slot and is warned about conflicts}}
# mod/scheduler/tests/behat/conflicts.feature:31
{{ And I add 5 slots 5 days ahead in "Test scheduler A" scheduler and I fill the form with:}}
# mod/scheduler/tests/behat/conflicts.feature:35
{{ Link matching locator "'Add slots'" not found (Behat\Mink\Exception\ElementNotFoundException)}}

The 'Add slots' as a drop down needs a fix.