Assuming that the XForms namespace has been defined with the prefix xf, setting all XForms input controls to have a blue background would require the following style rule:
xf\:input
{
background-color: blue;
}
Note that unlike the 'proper' namespace support defined in CSS 2.1, the prefix used here must exactly match the prefix used in the mark-up since all that is happening is the element called 'xf:input' being matched.