Ok part of the solution is to do the following.
Go to System > System Settings.
In ’Filter By area’, select ’Rich Text Editor’
In ’Path to CSS’ , put the path to your CSS file. In the example it has a slash at the beggining of the file path,
eg. ’/assets/site/style.css’
But I found the path worked properly without the beginning forward slash, ie:
’assets/site/style.css’
I think there is a way of customising which styles appear, but I havent got my head round that part yet - Im going to make a limited custom CSS file and link to that instead