There are probably 10 ways to do this, and all of them force you to fight what the page wants you to do. Below is the more straightforward way to do it. To really have it all “centered” absolutely would probably require wrapping the markdown tables in html DIV tags and associating even more CSS, which … Well, you suggested you wanted a relatively simple solution, so here you go!
Try this — stick this in the markdown entry directly at the top, or in the code-injection (header) section in the per post settings (where you set the header for your post, etc). Note, not all computers have “Courier” as one of their installed fonts (it’s proprietary), so I pulled in a free font from Google (“Cousine”). “Liberation Mono” is another good option. If you want, you can move this stuff into the site-level code-injection settings, just be aware it will effect every post if you do that.
<link href="https://fonts.googleapis.com/css?family=Cousine&display=swap" rel="stylesheet">
font-family: "Courier", "Cousine", "monospace";
Play with those numbers. It depends on your theme, etc.
Here are two postings that of the same thing that shows you what they did to my tables. (mind you, my site is somewhat heavily CSS manipulated to begin with).
Before widening the tables:
After widening the tables:
(I will be deleting this post in a week or so)