Add header/footer variation for white text on charcoal-1

[bengreeley]

The Enterprise website needs the charcoal-1 background color to be set in the header and footer, which currently isn't supported.

Created PR to add variation. @ryelle @StevenDufresne let me know if this needs any changes. It works well for me locally, but my local has had a few gremlins, so I'd love a second set of eyes.
#307

[ryelle]

Just for consistency, I'd like to confirm that this is really needed from design — for example, the download counter is also a dark page, but it uses charcoal-2 as the main background, which matches the default dark header. Is it possible Enterprise could also use this, or do you think we'll need a charcoal-1 header for more pages? @jasmussen @javierarce

Here's a comparison of the existing dark header + background combo on the download counter.

Download Counter	Enterprise

Edit: What I'm trying to avoid here is an arbitrary set of header colors, all slightly different.

[jasmussen]

I would think using charcoal-2 can work, but I'd let @panchovm confirm.

[fcoveram]

Let's use the existing dark header. The charcoal-1 is a mockup error. Thanks for the ping.

[ryelle]

Thanks! I'll close this issue then.