You can change the look of a table by changing its outline, showing or hiding gridlines, add borders and color to cells, and using alternating row colors.
You can also add a border around your sheet. See Add a border around a sheet.
Go to the Numbers app on your Mac.
Open a spreadsheet, click the table, then in the Format 
sidebar, click the Table tab.
Do any of the following:
Change the outline: In the Table Outline section of the sidebar, use the controls to change the line type, thickness, and color.
Change the gridlines: In the Gridlines section of the sidebar, click the buttons to show or hide gridlines from the table body, from the header rows and columns, and from footer rows.
You can change the border and background of any selection of table cells that don’t use categories.
Go to the Numbers app on your Mac.
Open a spreadsheet, then select the cells you want to change.
To change all the cells in a table, select the table.
In the Format sidebar, click the Cell tab.
Do any of the following:
Change the cell borders: In the Border section of the sidebar, click a border layout button below Border to choose which borders you want to format. Click the Border Styles pop-up menu, then choose a border style. Or, use the controls below the menu to define your own style.
Tip: To select multiple borders, click a border layout button, then Command-click additional borders.
You can change the outline of a table in the Table section of the sidebar.
Change the background: In the Fill section of the sidebar, click the color well, then select a fill option. See Fill shapes and text boxes with color or an image.
You can also create rules to automatically change a cell’s look when the value in the cell meets certain conditions. See Highlight cells.
Go to the Numbers app on your Mac.
Open a spreadsheet, then select the table.
In the Format sidebar, click the Table tab.
In the sidebar, select the Alternating Row Color checkbox, then click the color well to select a color.
Row colors don’t alternate for header columns. Header columns have their own look based on the table style you choose.