Fix toggling of full-screen button

1. Accidentally broke it when renaming CSS classes
2. Also rename images to match the fact that darker is the new default theme.
Changes after review.

1. Fix enterFullscreen.svg to be white instead of black.
2. Rename `button` property on new widgets to be element, for consistency (and they are not all buttons).
Merge pull request #606 from AnalyticalGraphicsInc/fixFullscreen

Fix toggling of full-screen button