At the time of writing this, I'm using the Twentyfourteen theme which is the one I prefer.  However, I wanted it to use as much of the screen estate as possible.  I didn't like any other theme that had this, so I looked into modifying what I had.  Here is what I did...

Step 1) Go to your site admin page.

Step 2) Go to "appearance" and select "theme"

Step 3) Select the theme you wish to change and activate.

Step 4) Go to "appearance" and select "customise"

Step 5) Click the "Additional CSS" option at the bottom of the left sidebar.

Step 6) Add the CSS sections of what you want to change.

To achieve the look I wanted, I added the following:

.site {
background-color: #fff;
max-width: 100%;
position: relative;
}

.site-header {
background-color: #000;
max-width: 100%;
position: relative;
width: 100%;
z-index: 4;
}

.site-content .entry-header,
.site-content .entry-content,
.site-content .entry-summary,
.site-content .entry-meta,
.page-content {
margin: 0 auto;
max-width: 90%;
}