Have you tried modifying the theme style you are using in the form? The files are located in C:Program Files (x86)K2 blackpearlK2 SmartForms RuntimeStylesThemes, search for the table tag and add the border-color:  

Hi,

If you do that, does that mean that all of table in other smartforms or views will have border?  What if I only want to have a border for this particular table? 

Hi JudoK,

I believe the style will be applied on all the tables you have, the thing is you can't target one single table

Why not make a duplicate of the theme, and just apply the theme just for that form?