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 even lines.