Set background-color on .wh-footer

Set `background-color: #FFFF;` on `.wh-footer` to prevent the side-bar from being viewed through the footer.

Before
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![image](https://f.cloud.github.com/assets/598803/1424825/ef5a464c-4026-11e3-8017-bd9325c03537.png)

After
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![image](https://f.cloud.github.com/assets/598803/1424844/4c27b602-4027-11e3-87bb-a4f5136b2afc.png)