Hello,

You have set “full-width” in the menu item where you set this HTML block.

There are two ways to solve:

1. You can choose “Set-size” and set a fixed size for the block but it would not be centered.

2. We can make this background transparent and you will see you HTML block in the center as it is now without this white background, like this: https://gyazo.com/e9767f58d919f978a4b5d3c465db851e

As you see another block also got transparent, you can set the background color in the HTML block or assign the class to Friedenskonzept menu item, and I will give you CSS for this block only.

If you prefer setting the white background for bog mega menu block, you can use this CSS:

html .wd-nav.wd-nav-main > li .wd-dropdown {
background-color:transparent;
}

Please add the code to the Theme Settings > Custom CSS.

Best Regards