[addon background] Not persisting background color

[hally9k]

Bug

The background color flickers and reverts to default on 3.3.15

Steps to reproduce

Boot up 3.3.15 with background addon and attempt to set a background. The background attr gets set on the iFrame's inline styles but doesn't show. If i manually set it on the html tag inside the iframe it works. Might be a chrome thing. In Safari it is just a fine line at the top, see below, the intended background color is that hideous violet.

Please specify which version of Storybook and optionally any affected addons that you're running

• @ storybook/react 3.3.15
• @ storybook/addon-backgrounds 3.3.15

Affected platforms

• Chrome 66, macOS 10.13.4

[Hypnosphi]

It was fixed with #3331
Please try version 4.0.0-alpha.4