15153 customize styling browseable rest api

[arthanson]

Fixes: #15153

Limits on what can be changed without copying and modifying the base template into api.html, but made tweaks to bring it more in-line with app. Just kept inline css as fairly minor changes needed and IMHO not worth tooling for separate css file.

[arthanson]

Changed to dark header and adjusted spacing / alignment of items

[jeremystretch]

We're going to punt on this one for now and revisit once we've spent more time improving the overall UI design.