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filter is not described

wp_get_custom_css filter-hook . WP 4.7.0

Filters the Custom CSS Output into the <head>.

Usage

add_filter( 'wp_get_custom_css', 'filter_function_name_2777', 10, 2 );
function filter_function_name_2777( $css, $stylesheet ){
	// filter...

	return $css;
}
$css(string)
CSS pulled in from the Custom CSS CPT.
$stylesheet(string)
The theme stylesheet name.

Where the hook is called

wp_get_custom_css()
wp-includes/theme.php 1711
$css = apply_filters( 'wp_get_custom_css', $css, $stylesheet );

Where the hook is used (in WP core)

wp-includes/customize/class-wp-customize-custom-css-setting.php 88
add_filter( 'wp_get_custom_css', array( $this, 'filter_previewed_wp_get_custom_css' ), 9, 2 );