OK, try doing it this way:
Edit the HTML Editor instance. Click the Pencil icon next to the preset to edit the preset.
In the styleing tab, select the option to edit the styles listed in the menu.
you can select to create new styles, when you give thje new style a name, you can select that it applies to the table tag, the tr tag or the td tag.
in the bottom section, you can define the style attributes like border or font size / color.
after adding the table in the editor, select it, then from the style menu select the stye that was defined for the table.