It is possible to use external CSS files by importing them as solution's media files.
By default Servoy 7 mobile uses the default theme from JQueryMobile 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
As Servoy 8 uses JQueryMobile version 1.4.2, you need to use the corresponding version in the ThemeRoller, and import the CSS in the
solution's media with the name jquery.mobile.theme.css and the icons CSS with the name jquery.mobile.icons.css
*Be aware that changes will be seen only in the client, not in the form editor.