Broken layout. Request for quick help!

Hi guys, first of all, I have to give you a huge compliment! The usability of your plugin is just gigantic. And easy for everyone to understand. I’ve designed a new e-mail layout for a very long time now. Everything is perfect in the preview. But the final emails look different than my actual design. It does not take a background color in the header. The round corners, shades, etc. are also not displayed. I read that others had similar problems here. But unfortunately there was no solution. I ask you for quick help please!

Thanks.