How would i add a background to my forum and increase the size of the forum category
like this website rpland.org
You have to edit the body background in your theme's css. For the category size, you can click inspect element on it to find where the css for it is located and then edit your css file.
What is the line of code i need to edit?
That will depend on what theme you are using.
the core theme
index.css line 22:
body
{
background: /* add your background here */;
font: 95%/90% Verdana, Helvetica, sans-serif;
margin: 0;
padding: 12px 0 4px 0;
}