Styling adjustments for proofread view and toggle buttons

Several CSS adjustments for the toggle buttons (list and proofread)
and the layout of proofread view. The changes include:
* Adjusting the style to make them consistent with other buttons (.button class is now used)
* Use of images (in png and svg formats)
* Positioning (adding paddings to some elements)
* Typo fix for "width"

Change-Id: I1a6bcdca0a9c058663ad8ca273350613c7af42c8