Don't hardcode colors in Signup and Header cards

In the beta editor the Signup and Header cards have hardcoded colors for everything what makes it for a theme developer hard to customize.

I would suggest that there is a .kg-style-light, .kg-style-dark, .kg-style-accent (as it’s in the current Header card in the main editor) that don’t have hardcoded colors on them so theme authors can then apply their custom styling for them.

.kg-style-custom could be used for when there is a custom color which then is hardcoded. (although the class isn’t really necessary).

When a custom color is used instead of applying color to text it would be better to have something like .kg-has-dark-text and .kg-has-light-text on the card and the same class on the button.