Trouble with plaintxtBlog theme, simple answer?

Hi everyone!

Im having some difficulty in finding the correct css value to edit in this theme.

What I want to do is make the posts 400px wide, this will leave some white space to the right of the posts. I want the border to have the same padding as on the left, so that when anyone resizes their browser it stays the same.

https://themes.wordpress.net/columns/3-columns/60/plaintxtblog-20/

thanks!