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.

Brian Harris

This plugin adds support for the Twenty Fourteen theme to the <a href="https://wordpress.org/extend/plugins/styles">Styles plugin</a>.
Features of the plugin
<ul>
<li>Instant previews</li>
<li>Text size</li>
<li>Google Fonts</li>
<li>Text colors</li>
<li>Border colors</li>
<li>Background colors</li>
<li>Hundreds of options</li>
<li>Consistent interface and names in every theme</li>
<li>Built on WordPress customizer, so Styles grows as WordPress grows</li>
</ul>
Styles and options for all built-in WordPress themes are free. More themes are available at <a href="http://stylesplugin.com" rel="nofollow ugc">StylesPlugin.com</a>.
Contact
<ul>
<li><a href="http://wpdefault.com/forums/forum/styles-twentyfourteen/" rel="nofollow ugc">Get support</a></li>
</ul>

Styles: TwentyFourteen

Be creative with colors and fonts. Styles changes everything.

Add Background-Size to Customizer vs Styles: TwentyFourteen