No need to hold up this pull request for a naming nitpick, but "admin_css" is a bad name for this.  Since this is a base which expects its inheritors to be able to add new css, naming this "admin_css" basically implies that only admin sub-templates will ever do that.  Which is true right now, but the next time we want to make a sub-template which adds css, we'll be using a name which actually implies the wrong thing.  Better to just call this block "additional_css" or something generic.