Add background image option to createTheme()

[ethan-vanderheijden]

In all READMEs and the theme editor page, I added backgroundImage: '', to show people that the option exists. I don't think we need a background image customizing option on the theme editor page (a text box would look weird next to the color selectors), but let me know if you think otherwise.

[jaywcjlove]

@Ethan-Vanderheijden I agree with this perspective. Adding a background image customizing option next to the color selectors on the theme editor page might not be visually appealing. Thus, I agree that it is unnecessary to include a text box for background image customization on the theme editor page.