You adjust the table's properties. I don't have Thunderbird here right now to check how to do it, but try right-click, double-click, or selecting the table and then going to Insert|Table. Or it may be under Format or Options.

When you have the properties open you can set the table border width. I don't think it offers any options for styling the table. If that's what you want you may have to wrangle the html/css code.

Hi, Thank you for your reply. I found below: Click table > Format > Properties > Cell