This code:
if( get_theme_mod('enable_content_background_color') && empty( get_theme_mod('content_background_image') ) ) {
$custom .= " .site-content {
background-image: none;
}"."
";
Triggers this error:
Can't use function return value in write context in /home3/*******/public_html/*******/wp-content/themes/outliner/includes/styles.php on line 41
I wonder why?