Lesson

Can someone please shed some light on this for me - how do I use my own custom css styles in my lessons? I have tried adding my css to the existing scss files for the theme I'm using (Boost) but no matter where I put the code it does not affect my lessons pages. I also added a .scss file to the /moodle directory with all the other .scss files, then added an import line to the moodle.scss file (again, the one if the Boost directory), but nothing works. This is the 21st century and there has to be a way to use our own custom css styles in lessons, right? Isn't that the point of open source, so we can do what we want to do, and not be restrained by someone else's ideas of what we want. Well, seems to me to be that. But anyway, how can I use my own custom .css in my lessons? Or possibly in books? (BTW, this is a brand new install of the latest version of Moodle, and I am a new user)