Add new header variants

[thatbudakguy]

As of this writing, there are three variants that can be used for the header:

Dark

• The navbar and masthead have a dark background with white text.
• The exact color of the navbar/masthead background can be customized per-app (e.g. earthworks is green).
• The stanford identity bar at the top has a light background with cardinal red text.
• The SUL logo in the navbar is monochromatic white.

Light

• The navbar and masthead have a light background with black text.
• The stanford identity bar at the top has a cardinal red background with white text.
• The SUL logo in the navbar is monochromatic black.

White / "SDR theme"

• The navbar and masthead have a white background with black text.
• The stanford identity bar at the top has a cardinal red background with white text.
• The SUL logo in the navbar is polychrome; the Stanford wordmark is cardinal red and the rest is black.

Hopefully we can make this a little easier to implement by adjusting the CSS and updating our example page.