Once clean theme becomes the default one, the test suite will begin using it; before integrating it (2.5 release) I ran the whole suite using it and there were a couple of errors regarding block docking and another one that I don't remember.

Before clean becomes the default theme in master this issue should be done, and, depending on the changes it requires integrated before or at the same time than the issue that makes clean the default theme.

There will be cases where we will have to choose to change a xpath to bootstrap CSS selectors or maintain compatibility with both; as long as the compatibility with both themes does not imply a significant performance loose I would keep compatibility with both themes inherited from standard and bootstrap for a couple of major releases, so we can run the suite in different themes if we are interested on it.