Enhance custom background handling

[deshack]

RECOMMENDED: No reference to add_theme_support( "custom-background", $args ) was found in the theme. If the theme uses background images or solid colors for the background, then it is recommended that the theme implement this functionality.

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[deshack]

As of v1.1.x, we're not really using the custom-background feature. We just have some customizer options that customize the static front page colors.

In order to support the custom-background feature, we should refactor all of this, with respect to existing settings, and add the ability to customize the background and color of all pages. This is not that easy and requires at least a new minor version.

More on this in the Codex and in the code