To demonstrate the possibilities with icon-fonts, some text-shadow and transition effects are applied.
Be sure to use only icons that are available in your map.apps version. The availability of Esri icons depends on the version of ArcGIS Maps SDK for JavaScript included in map.apps.
Can be used out of the box (version 4.0.0)
map.apps provides additionally prepared css classes that can be used by adding "material-icon-" to the icons name. E.g. to use the icon "Sports Score" as toolIcon simply add `iconClass: "material-icon-sports_score"` in the tool's configuration.
Be sure to install the bundle mapapps-fontawesome before using icons from this icon set.