Thanks for the reply Tim,
I do agree that our features are not well written and are not inline with BDD philosophy. While working on MDL-55070, we will try not to have specific features for the alternate theme but to fix the existing features and override context and not use specific steps like "I click on xyx" .
With huge number of features in core, I am not sure if it would be possible to get all features fixed in MDL-55072 (Adding support for alternate theme). Also, some of the features might not be valid for alternate theme (Themes not supporting blocks), so the suggestion to blacklist features/override will be needed.
Adding more selectors and it's usage is surely a way to go forward and we should be able to let theme/plugin to add/override selectors. I will add that as part of the specs.