update background & text color placeholder (#34)

Co-authored-by: Louise Devolder <louise.devolder@onlyhumans.com>

src/helpers.php

@@ -11,7 +11,7 @@ function get_placeholder_url_by_format($format)
         $height = $format?->height();
         $name = $format?->name();
 
-        return "https://via.placeholder.com/{$width}x{$height}/21348c/ffffff.webp?text={$name} {$width} x {$height}";
+        return "https://via.placeholder.com/{$width}x{$height}/edeced/edeced.webp";
     }
 }