Your code appears correct but you lack the a tag in the selector so try modifying it to this .fl-builder-content .bb-mmenu a.
Our apologies for not having the option to easily change the font in the module, it does sound like something we should be considering. Feel free to add that in as a feature request on our UserVoice.
Why do I need the “a” - can you point me at a resource on how to address custom CSS ? Sorry beginner questions but if I can get my head around custom CSS for BB I think I can do anything I need to!
When dealing with CSS, you need to be familiar with precedence. Most of the time there’s no wrong code, there’s just another one that takes precedence. For instance, you had a style applied on an li which should work if a text is directly inside li with no other tags but if there is either a p or an a tag wrapped around the text, styles applied on the a/p tags will take precedence than you’re styling on the li.
When working with CSS, the dev tool is your best friend. You can try out different codes using the dev tool alone. I personally use Chrome’s dev tool(press f12 to launch) but you’re free to use any browser you like.