Add Background Toggle

[CraigCecilSSA]

When HTML elements don’t have backgrounds, it can be difficult to visualize their bounds and/or accurately measure the distance between them and other elements. Developers might want to better visualize bounds when dealing with optical imbalance (i.e. when something “looks off” even though it’s not), margin collapse (when certain margins are ignored), various issues with display:/float:/position:, and more.

Applying backgrounds means applying a semi-transparent background to all HTML elements in order to better visualize their bounds and spacings.

See https://css-tricks.com/web-development-bookmarklets/#applying-a-background-to-everything for more info.