Update theme to match ASF's new website

[jhkennedy]

When this theme was created, we matched the theme to the website so users likely wouldn't notice jumps between asf.alaska.edu and team docs pages.

The ASF website has since undergone a revamp and we should update the theme to match.

[jhkennedy]

Word on the street is there may be more changes coming down the line and no one is sure how long the current theme will stick around, so let's hold off until the main website settles.