cmborchert

This plugin for WordPress adds a background-size controller to the Background Image section in the WordPress Customizer.
You can access the new controller by going to Appearance > Customize from the Dashboard, and clicking on the Background Image section.
Once you’ve added your custom background image, you can set your background-image size to: 
 – “Cover”: As large as possible; fills entire view, with cropping. 
 – “Contain”: As large as possible, without cropping; fits inside of view. 
 – “100% width”: Width fits view perfectly, height may be smaller or larger than view (equivalent to css ‘100% auto’). 
 – “100% height”: Height fits view perfectly, width may be smaller or larger than view (equivalent to css ‘auto 100%’). 
 – “Inherit”: Sets background size to whatever parent element is set to (html, in this case). 
 – “Custom”: Allows you to put in your own css to be applied in the section called “CSS for background-size (if ‘custom’ selected).”
To learn more about the CSS background-size property, go to:
https://developer.mozilla.org/en-US/docs/Web/CSS/background-size 
or 
http://www.w3schools.com/cssref/css3_pr_background-size.asp
<h3>Notes</h3>
Notes: 
*The plugin works by adding styles to the body.custom-background class. If the default custom-background support has been disrupted, then this plugin will not work.

Add Background-Size to Customizer

Allows you to control the background-size of your main background image from the comfort of the customizer, using predetermined values or custom CSS.

Enej Bajgorić

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Add Background-Size to Customizer vs Improved Simpler CSS