Like @Festarossa said, the 404-ing background image is specified in the stylesheet:
header.d-header {
padding-top: 40px;
padding-bottom: 40px;
background-color: #002040;
background-image: url("https://cdck-file-uploads-global.s3.dualstack.us-west-2.amazonaws.com/flex030/uploads/thimbleweedpark/original/1X/0c33436eebb85e247437fe1c7ac01e5ba510b840.png");
}
There are various methods to override that with your own desired values. Stylish is a popular extension for browsers that don’t have the built-in ability to do so.
Anyhow, so you could override the background image with the URL from this post: