Description 
 If installing the ‘wp-admin/’ directory will be replaced with ‘./’.
 The $_wp_admin_css_colors global manages the Administration Screens CSS stylesheet that is loaded. The option that is set is ‘admin_color’ and is the color and key for the array. The value for the color key is an object with a ‘url’ parameter that has the URL path to the CSS file.
 The query from $src parameter will be appended to the URL that is given from the $_wp_admin_css_colors array value URL.
    Parameters 
  - $src
-   (string) (Required) Source URL.  
- $handle
-   (string) (Required) Either 'colors' or 'colors-rtl'.  
Return 
 (string|false) URL path to CSS stylesheet for Administration Screens.
    Source 
  File: wp-includes/script-loader.php 
 function wp_style_loader_src( $src, $handle ) {
	global $_wp_admin_css_colors;
	if ( wp_installing() ) {
		return preg_replace( '#^wp-admin/#', './', $src );
	}
	if ( 'colors' === $handle ) {
		$color = get_user_option( 'admin_color' );
		if ( empty( $color ) || ! isset( $_wp_admin_css_colors[ $color ] ) ) {
			$color = 'fresh';
		}
		$color = $_wp_admin_css_colors[ $color ];
		$url   = $color->url;
		if ( ! $url ) {
			return false;
		}
		$parsed = parse_url( $src );
		if ( isset( $parsed['query'] ) && $parsed['query'] ) {
			wp_parse_str( $parsed['query'], $qv );
			$url = add_query_arg( $qv, $url );
		}
		return $url;
	}
	return $src;
}       Changelog 
     | Version | Description | 
    | 2.6.0 | Introduced. |