Try this on blank new form ..
1- drag radio buttons to the form, make them 2.
2- save source files, close the form
3- create css in source files folder, put this in it :
.xdBehavior_Boolean
{ background-color : black ;
}
save your css as test.css
4-open view1.xsl, exactly after <head> tag put this :
<link rel="stylesheet" type="text/css" href="test.css"/>
save and close
5-right click your manifest.xsf, click design, click upload text.css as resource file, save and close form , re-open manifest.xsf in design and see the results.