So you gave the layout grid’s class-property the value ‘tableborder’. That is step 1, it should show up in the browser's inspector as a class of your layout grid-element.
This CSS defines the shown border to have a white color and a white background, so it is there, just not visible. Solve the problem by removing the “!important” at border-color or better replacing it by border-right and border-bottom, which will be less confusing.