1) I checked your header and it’s white, I don’t see any black bg for the mobile version https://gyazo.com/92bf086143e760e95be91c055630c127
Provide a screenshot or video of the issue for a better understanding, please.
2) Error appears because of yith-woocommerce-order-tracking-premium plugin and show be fixed in plugin files. So, please, contact plugin author about the mentioned error. If the plugin is disabled I don’t see any issue with the builder.
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.