Improve accessible color contrast validation

Currently, a theme will validate with a color contrast between 3:1 and 4.5:1. When you are editing your theme, you will see these in a yellow "warning" color. Any color combinations in this range should only be used for large text, over 18px and bold or over 24px. Most fonts in Aspen are not this large, and using and color contrast that appears as a warning will fail WCAG 2.0 Level AA accessibility standards.