.sidebars #primary, .sidebars #secondary, .content-wide-sidebar #primary {
    border: 1px dashed gray;
    padding: 5px;
    width: 250px;
}

The border attribute is (surprise, surprise) the border - one pixel wide, dashed, grey. You can change all of these. As many pixels as you want (one seems to fit your overall look); I chose "dashed" for the sample code, but you can choose between it, dotted, and solid. You can define a hex value color instead of a predetermined shade (it would look like border: 1px dashed #0F0F0F; for example).

You can also change the border attribute to "border-left" and it will only show up on the left side of the sidebar, making it more like a dividing line. I would increase the padding (to 8px or higher). It's cleaner in my opinion, but whatever you ultimately choose will look fine.

Good luck!

#header-message {
text-align:center;
color:#000;
}

Any ideas on how to put a border around either the widgets or the posts? There's nothing but some white space to separate the two.

That said, I'm guessing your tag line is set in a div with a CSS selector. Mine is #site-description, so I'll use that for my example.

You want to center the div, not just the text within the div, so set a width for your div (it may already have one; that's fine) and then set the left and right margins to "auto." Something like this:

#site-description {
  width: 250px ;
  margin-left: auto ;
  margin-right: auto ;
}

Good luck!