If you grab the right edge of your browser window and narrow it down, you will see that the theme you are using adjusts for the size of the browser window. Confit is a responsive design that adapts to the size of the browser window/screen it is being viewed on. This makes it difficult to "center" the content in the black area since the width of the black area depends on how wide the browser window/screen is.
It may be that it could be centered to where it would adjust with the width of the screen, but then on very wide monitors, you would end up with a large gap between the sidebar and the content. This would all have to be done quite carefully to keep the responsive design working correctly and not have it blow up on smartphones.