refactor a11y-behaviors to account for options in simple colors

[btopro]

https://github.com/LRNWebComponents/a11y-behaviors/blob/master/a11y-behaviors.html#L18 more specifically. It would be good to validate this function and ensure that whenever a color is selected for background that a safe text color can be requested via function. video-player and other tags that implement the accessible color swap capability will allow the user to pick a color and then calculate what text color to use. Some manner (either this function or just a simple class alternating based on light / dark theme) should be enough to ensure that this happens.