If you have a long and wide table, it can be hard to follow the lines with your eyes. Therefore tables are often made zebra striped with a different colour for the odd or even lines. When working with the table in Site designer, you won't see the different-coloured lines, but you will in preview.
To add rows to the table just copy one of the existent rows, class name .table-row as many times as needed. You don't have to worry about the colouring of lines, because the styling will automatically give the different colour to all odd lines.
Instead of choosing the odd lines, I might as well have gone for the even ones.