how would I change background color of a theme using a plugin?

Question

I would like to change themes background color using a plugin. For when I use this code,

function change_background_of_theme(){
?>
<style>

#main{background: yellow;}

</style>

<?php
}
add_action('init','change_background_of_theme');

This code only changes the bg color of the theme that use #main id. This basically leads to another question, is there a common id that themes use for background or something close to that. I appreciate your assistance. :)

-Ron.

Answer 1

If the theme uses the custom-background functionality of WordPress, then WordPress outputs CSS code in the head which essentially says body.custom-background { ... } and it puts the chosen colors and image in there.

So most themes sorta expect that to be on the body itself, and to have a custom-background class on the body tag.

Answer 2

If the plugin uses the custom-background, then you can switch easily, updating the "theme_mods_{$name}" option:

/** Plugin Name: (#64373) Custom background Color */
function wpse64373_set_custom_background( $new_colour )
{
    $old_colour = get_theme_mod(
         'background_color'
        ,get_theme_support( 'custom-background', 'default-color' )
    );

    // Nothing to do here
    if ( $old_colour == $new_colour )
        return;

    return set_theme_mod( 'background_color', $new_colour );
}
function wpse64373_update_custom_background()
{
    // Not sure if we have to prepend a `#`
    wpse64373_set_custom_background( '009EE0' );
}
add_action( 'init', 'wpse64373_update_custom_background' );