While the (default) CSS has a maximum width for the text, the resulting block is shown left-aligned. Look into making it centered instead.
Having looked into things, markdown.css actually centers the complete document. But it left aligns the text body and the headers in the document, while the headers can grow to fill quite some size, the body though being restricted to a smaller size. Which results in the text body to be not centered to the viewport usually. I picked markdown.css to avoid getting into the business of doing CSS, as I lack designer knowledge. So instead of starting random changes to the original design, any future effort should rather go into solving https://bugs.kde.org/show_bug.cgi?id=385219 (Support custom stylesheet), so everyone can please themselves as they prefer.