It is possible to use external CSS files by importing them as solution's media files.

By default Servoy mobile uses the default theme from JQuery Mobile version 1.3.2; you can overwrite that, with a custom
theme (created with ThemeRoller https://themeroller.jquerymobile.com/ for version 1.3.x ) by importing the CSS in the solution's media
with the name jquery.mobile.theme.css