You’ll definitely want to clean up some of those errors. Simply paste the custom CSS you’ve added yourself into this input: https://jigsaw.w3.org/css-validator/#validate_by_input
That will make sure it only scans your custom CSS.
To make it like that site, you need to:
1. Save the title/logo as a different image file with no background (transparent PNG).
2. Save the background image as a different image (without the title).
3. Upload the background image in “Customize > Background Images”.
4. Upload the title/logo in “Customize > Site Identity”.