You can add a border, such as a picture frame or a dotted line, around an image, shape, text box, or video, then modify the border’s thickness, color, and other attributes. You can also use a shape to create a border around a sheet.
Note: To add or change a border for a chart, see Add a border and background to a chart. To add or change a border for a table, see Change table gridlines and colors.
Click to select an image, shape, text box, or video, or select multiple objects.
In the Format 
sidebar, click the Style tab at the top of the sidebar.
Click the disclosure triangle next to Border, then click the pop-up menu and choose a type of border (line or picture frame).
Use the controls that appear to adjust the look of the border.
To add a border, such as a solid or dotted line, around a sheet, you first add a square shape to the sheet, then change the shape’s attributes so it behaves like a border. For example, you change the size of the shape, give it no fill (as opposed to a color fill) so that it doesn’t obscure other objects on the sheet, and so on.
Click 
in the toolbar, then select the square or rounded rectangle from the Basic category.
Drag the white squares that appear around the shape until the outer edges of the shape form the border size you want.
In the Format sidebar, click the Style tab at the top of the sidebar.
To remove the color from the shape or choose a different color, click the pop-up menu below Fill, then click No Fill or choose a different fill option.
Click the disclosure triangle next to Border, then click the pop-up menu and choose a type of border (line or picture frame).
To change the transparency, drag the Opacity slider.
To lock the border so it doesn’t get moved accidentally, click the Arrange tab at the top of the sidebar, then click Lock.