Table formatting seems to be a struggle for a lot of Zendesk admins/agents. We designed our own Zendesk Help Center theme and had such a headache with the tables that we added default table formatting to the CSS file and can just override horizontal alignments and add any simple text formatting (bold, italics, etc.) and such in the WYSIWYG editor, as needed.

I've added the code we use for our table formatting below, if helpful. Adding that padding line (where the arrow is pointing) helped prevent our table details from running into the borders. It was added to the "td" section.

Now this is how our tables look in the WYSIWYG editor (note that I did change the first row to be a Header row from within the WYSIWYG editor and kept the rest as Body rows):

And this is how the same table looks when it's published:

Hope this helps someone having issues. We've actually appreciated the standard formatting so our articles look consistent. However, it would be nice to have some better variation from within the editor.

UPDATE IN JUNE 2020

@... posted a Feature Request for this at the link below.

https://support.zendesk.com/hc/en-us/community/posts/360046933853-Feature-Request-Make-editing-Default-Table-Formatting-for-Help-Center-articles-easier

If you would like to see this feature please head over there and show your support. Make sure to add an upvote and comment even if it is simply a +1.