Is there any way to change the background of the my website in Discourse

Development
2

The first thing I would do is use the browser inspector. That will allow you to see the HTML and the CSS they are employing.

Without getting too much into the weeds here which is mostly generalised CSS, a simple solution is to do something like:

body {
  background-image: url($myimageurl);
  background-repeat: no-repeat;
  background-position: center;
}

They seem to be doing something similar.

Make sure you’ve read through all the relevant articles in here: https://meta.discourse.org/tags/c/howto/devs/56/themes