Hi Christian,
In the iframe code does the url include OnePane/basicviewer? If so then you can't customize the font but you can choose from a set of pre-defined color themes. The available colors are orange, green, blue, gray and purple. To change the color you'll need to add a theme parameter to the url. Here's an example that shows the green theme.
src="http://jsapi.maps.arcgis.com/home/webmap/templates/OnePane/basicviewer/embed.html?webmap=106d8ef38ae..."