You can change image border thickness and color using the Design toolbar.

To format image borders

Select the image whose borders you want to format.
On the Design toolbar, use the buttons in the Text & Color section to format the border.
To modify the thickness of the border, click the Border Size button. The Border Size menu appears and you can select a thickness between 0.25 pt and 6 pt.
To modify the color of the border, click the Select Color button. The Select Color menu appears and you can select a color.
- Automatic: Selects the default color for the border based on your selected theme.
- Theme colors: Offers a selection of colors from within your selected theme.
- Standard colors: Offers a basic selection of standard colors.
- More colors: Drops down a color picker.
To modify the border style, click the Border Style button. The Border Style menu appears and you can select a style.