The way the developer created the vertical line between the sidebar and the content was by adding the following on about line 1404 of your theme’s style.css:
.site-content {
border-right: 1px solid #F1F1F1;
}
The lines between the content items are from this, on about line 874:
.site-content article {
border-bottom: 2px double #F3F3F3;
}
Since the article is inside the site-content div, and you have an additional element, the nav tags with the ID “nav-below” that has the “Older posts” navigation link, below the article and above the end of the site-content div, that accounts for the extra length of the right border.
You have a few options. One would be to try moving the “nav-below” nav tag outside of the div with the ID “content” and see if that’s enough. You’ll may have to tweak the CSS of the “nav-below” element when you do that.
The next option would be to add a div to enclose everything in “content” except that “nav-below” element, add a border-right for it, and remove the border-right for .site-content. You would add padding-right to the new div to move its right border out to where you want it.
You would need to make the change–the added div or moved nav-below–for all your theme’s pages; usually, that’s index.php, single.php, and page.php so that it works on all the pages of the site.
It’s a 15-minute job for a developer, but if this HTML and CSS looks like Chinese to you, it’s going to be a chore. It’s worth learning, though.
In any case, you really should create a child theme first and work with it so that your changes aren’t overwritten if you ever update the parent theme.
