ok so this one is easier but anyways this is how to do it:
find image (usually right click > copy image address)
then type this in:

body {
background-image: url("link");
}

type(or copy) the link to the image(image address) into the place where it says: link

update:
to make it work cooler do this:

body {
background-image: url("link");
background-repeat: no-repeat;
background-position: center;
background-attachment: fixed;
}

type(or copy) the link to the image(image address) into the place where it says: link
(my current background like (copy address): https://art.ngfiles.com/images/1785000/1785693_demoprofile_please-ignore-this.png?f1619773342)

since i cant find any tutorials here's how you do this:
so let's say i wanna make green background with the word green

body {
background-color: green;
}

that's what you do, but, if that doesn't work for you, there are other options
hex colors:

body {
background-color: #00FF00;
}

then we have:
red green blue

body {
background-color: rgb(0, 255, 0);
}

and of course
red green blue alpha (transparency) [mess with the 4th number for transparency]

body {
background-color: rgba(201, 76, 76, 0.3);
}

and yeah that's it