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Frame with Image around Website

I would like to have a frame around all content of my website. In addition there is a lettering at the top, which breaks the line of the frame at this point. See the image at the bottom for reference.

I have only found a way to have a responsive frame with borders for div-boxes, but the image does not resize properly on smaller screens. What would be the right way to do it in order to also make it responsive?

Thank you so much for your help!

PS: Here is the link to the project