This is happening because the black bar is part of the background image for the whole page, and all of those images are 1600 pixels wide and being applied via "legacy presentation attributes" rather than CSS, so you don't have any control over whether or not they repeat horizontally. A reasonable patch would be to add this attribute to every single BODY element on the entire site:

style="max-width: 1590px; background-repeat: repeat-y; background-color: black;"

That way, you still get a black bar at the far right, but it'll be as wide as it needs to be -- no second copy of the background graphic beyond it -- and the content won't stretch over the bar.

If it is easier to do it this way (frex if it can be done in one place rather than dozens), adding a STYLE tag to the HEAD with the contents

body { max-width: 1590px; background-repeat: repeat-y; background-color: black; }

would have the same effect (it's just a different way of writing the same thing).