Elm currently doesn't have template strings which is the reason I decided to implement each CSS declaration as a function. Maybe a way to support both would be great so it's faster to get into elm-styled. The problem is that inserting values into a String is pretty ugly in elm currently.
fontSize = 2
header = styledCss div
"""
font-size: """ ++ (toString fontSize) ++ """rem;
font-color: red;
"""
Maybe it's not worth to support that because elm-css is accepted very well in the elm community as well. In addition with this method you would lose the power of typed Css.