Hi Sarah,
Have you tried Joseph Rézeau's suggestion (LINK)? He tested the CSS in Moodle 2.7 with Clean. If it still doesn't work it may be helpful if you post a screenshot of the title of a lesson activity that you want to get rid off. Although I don't think that themes style the lesson activity it may also be good to know which theme you are using.
With kind regards,
Hartmut