Is there any guide or reference for text code for tables and so on for the current version of the forum?
You can use Markdown, BBCode, or HTML. See more discussion at: Post format reference documentation - #2 by codinghorror - faq - Discourse Meta
I tried these, the BB code doesn't work and I can't get the others to generate vertical lines.
[table]
[tr]
[th]A[/th]
[th]B[/th]
[th]C[/th]
[/tr]
[tr]
[td]D[/td]
[td]E[/td]
[td]F[/td]
[/tr]
[tr]
[td]G[/td]
[td]H[/td]
[td]I[/td]
[/tr]
[/table]
Note the link in the comments to Markdown Tables generator - TablesGenerator.com
| A | B | C | D | E |
|---|---|---|---|---|
| F | G | H | I | J |
This is what the code generates that claims it will have vertical lines- it doesn't.
If you can find a mod you'd like me to install, I'd be happy to. But I'm down to Google searches just like you, otherwise.
Both markdown and html tables do work here, even if BBcode ones don't, but you're right about the vertical lines. I suspect this is similar or identical to the issue brought up here: tables do not print vertical lines · Issue #129 · yzane/vscode-markdown-pdf · GitHub . If so, someone maintaining the site would have to change the table style the forum uses. The mods used are in the third post.
Thank you callen! Let me know if it's working now, or if you have any other CSS requests. 
Yes, when you changed that it fixed my post above
But it changed the formatting for all the lists of posts on the whole forum! LOL!
Okay, I made a separate class for tables with lines. To use it, add <div data-theme-table="lines"> before the table html code and </div> after it
| Item Name | Price |
|---|---|
| Book | $5.00 |
| Shoes | $10.00 |
| Flowers | $3.00 |
Well, yes, if you don't make exceptions. 
That sounds like a good way to do it. I'll note here, in case people get stuck trying, that this gives lines with html tables but not with markdown tables.