You can centre a table using CSS by setting the left and right margin for the table to 'auto' .
I'm currently on 2019.0.5 where there's a bug which means I can't set these through the UI. So, firstly try to type or select auto in the left and right margin fields. If you can, you should see the table magically centre on the page.
If you can't type 'auto', continue reading.
If you can't select or type 'auto; in the Margin fields (for example, in RH2019.0.5), you need to edit the code view:
1. Make sure you've selected the table tag correctly. 'table' will display at the top of the Properties pane.
2. Set left and right margin to 20px (or any number really).

3. Switch to code view. You'll see a table tag similar to:
<table cellspacing="0" class="table" style="margin-right: 20px;margin-left: 20px" width="50%">
All we care about are the margin-left and margin-right values.
4. Change margin-left and margin-right to auto:
<table cellspacing="0" class="table" style="margin-right: auto;margin-left: auto" width="50%">
5. Switch back to Author view and marvel at your centred table. Note that the margin values display completely wrong.

You can also do the same for specific table styles in your stylesheet so tables with that style are centred.