Add icons style

[ddelagneau]

Added rules for sizing the icon, icons as black on a light background and icons as white on a dark background.

[marella]

Hi, sorry for the late response and thanks for the PR. Currently this package only provides the minimum required css to load and render the fonts. Font size and colors are application specific so I don't want to include them in the main css file and currently there are no plans to include them in this package. In future if I decide to include them (in a separate optional file), I will let you know.