css: Explicitly specify units for font size
css: Explicitly specify units for font size

This rids of the following runtime warnings against latest gtk+:

(gnome-boxes:15902): Gtk-WARNING **: Theme parsing error: gtk-style.css:92:17: Not using units is deprecated. Assuming 'px'.

https://bugzilla.gnome.org/show_bug.cgi?id=691078

vbenes/gnome-boxes,vbenes/gnome-boxes