Simplify layout structure around .darken

Summary:
Footer and .darken has no padding margin defined, so the footer
doesn't need to be in darken (it has a plain color).

This configuration has been detecting trying Motion UI:
when the footer was fading out, darken class transparency
still appeared.

Test Plan: Check on Chrome and Firefox on several resolutions

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D553