If you’ve never seen SASS before that can be a little daunting, but what he is using is just basic CSS stored as variables. SASS contains math functions too, so a lot of that is being done for him, whereas if you were to do it, you would need to calculate the values yourself. The
@mixin part is obviously where all the work is being done.
If you read through the code, try and get an understanding of what he is doing. Fork it and start messing around with the code to find out what does what. Pull it apart, break it and fix it again. You will learn a lot, and if you manage to convert it to just CSS then you will have done a damn fine job :)