Cant find codes to change backgrounds and width…

God im so pissed, i just wrote the longest post and to learn about the tags I clicked it and now lost it all. You dont even understand. But anyway. This is what I need help with.

First check out my forum to get a reference point : https://www.damienbenoit.com/blog/

What I want to do is take off the border and make the whole forum just white, with that top banner still there cause I will change that later. And then I want to stretch the blog 100% so it stretches the screen width when sizing the window. I want the banner at the top to be at the left alignment with the post content as well, and the Sidebar all the way to the right like it is now but only stretching.

And then I want where the post titles and post category,edit,comments part to have a background color/box on them so you can easily distinguish between posts. Also I will make my forum have the same look as that and have them linked together like that.

I really tried and researched the theme support section along with looking through all my php theme editor files and still cant figure out how to accomplish this. Can someone please help me out? I can possible help you out by creating a design or something for you. Whatever services I give, ill help for your help. Thanks!