Looking at what you've said above the relative path for to load your CSS should be:
Each ../ represents a directory. It basically means move up a directory. So in your case you need to go up how many directories to get to the CSS folder and then get into it to access the style sheet? The Answer is two then access a folder in there.
So you need to go up two levels to access the CSS folder so therefore you need two ../ parts. Somebody could probably explain it better than I have but you should be able to use the code above to load your CSS.