Colour:
Add one of these tags to the page on which you want to change the colour of the form.
form theme: primary
form theme: secondary
form theme: tertiary
form theme: quadrinary
Gradient:
Recommended for a splash page.
layout_form_gradient
Alignment
Add one of these tags to the page you want to change the form alignment.
layout_form_left
layout_form_right
layout_form_center
layout_form_full