Where can you change the background that comes by default on every post. If it needs to be changed by code what will be the path?

That should be in the css of your template. If you really want to know where then in Chrome for example, you click right click on the background part and click on inspect element. This will give you the info you need as too what div, class and css file you need to edit.

Other browers might have some different developer tools but the goal is the same. just inspect the element and you'll find the answer. Since most template will used different name, folder and class it's hard to tell you exactly what file and where you have to edit the background.