Portals; Sign-In form buttons show white-on-white text if Theme Contrast Foreground Colour is White

On the Theme page of the Portals form, various colours can be defined.

If the Contrast Background colour is a light colour such as "white", this produced unreadable text on the Sign In button on the built-in Sign-In form.

This has been updated to use the Contrast Foreground colour as the button text.

The algorithm to display this colours also not detects if the Contrast Background Colour is NOT dark and if so, switches the colours on the Sign-In button to favour a light-on-dark colour scheme.