Fix floaring header gradient in FF and settings images

No issue

- Corrects the syntax used for the floating header gradient so it now gets prefixed correctly for Firefox
- Removes padding from `<button>` elements (fixes padding around images in buttons)
- Adds display: block; to settings images to prevent a Webkit bug which adds space under the image
Merge pull request #3952 from PaulAdamDavis/fix-what-libsass-broke

Fix floating header gradient in FF and settings images