Optional config for login screen background color gradients (independent of primaryColor)

[doc-rj-celltrak]

This change is to add an optional LoginTheme config for the login screen's light and dark background color gradients. If provided, the pageColorLight and pageColorDark color values will override the primaryColor and computed primaryColorDark values for the light and dark gradients used on the login screen background.

The reason is that you might want a lighter background color than what is used for your primary color in the form field backgrounds, etc. Also, when using a lighter color for your primaryColor, the computed primaryColorDark often doesn't actually provide a darker color, so you don't get any gradient effect at all. Anyhow, a lighter primaryColor can cause the form fields to blend into the white auth_card background, with not enough visible contrast.

So, this is just one potential way to give users more control with LoginTheme config options.