To change the style of your table, click on the Styles drop-down menu located at the bottom center of your slide. Here, you can customize your border, column and row lines, and much more.
Selected style options have a dark black border around the icon while deselected options appear greyed out. In the example below, Border, Row Lines, and Header Row & Col are selected styles while Column Lines, Left Cell, and Banded Rows are not:
To change the style of an entire column, click on the Column Header. Then, click on Column Style and select an option from the style list:
To change the style of an entire row, click on the Row Header. Then, click on Row Style and select an option from the style list: