Table Styles

@NicoleBergantino I have just learned there is an issue with nested tables in the Bug reports. It is expected that this will be fixed in the next update. Maybe that will help you.

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

Regarding the unfamiliar terms in the stylesheets, they're mostly all standard css, so I would recommend running through the css tutoral. You don't have to run through all of it, but can dip in an out to research specific terms as you come across them, but I do recommend at least doing the first 4 or 5 segments to get a feel for how the tutorial is structured and the basics of css.

https://www.w3schools.com/css/default.asp

Mozilla also has good information:

https://developer.mozilla.org/en-US/docs/Web/CSS

and I like their reference docs:

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference

2019 Classic or New UI?

If it was the New UI, I don't think any of the changes should have been that catastrophic. However, with either version I think I would look at recreating one of the tables in 2020 to see what breaks it as you set it up. Then you might have the answer as to what needs fixing.

Sorry it's not really the answer you want but it's one of those things where you have to be working on the specific topic, not one where there's a general answer. That is unless of course someone else has hit exactly this scenario.

Otherwise you can try Support. See https://helpx.adobe.com/contact/enterprise-support.other.html#robohelp for your Adobe Support options. The email link tcssup@adobe.com is recommended as it reaches a team dedicated to Technical Communication Suite products including RoboHelp. 

________________________________________________________
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.