The Stroke Tool can be used to adjust the stroke (outline) colour on Cakes and Boards.

Simply select a cake or a board, open the Stroke Tool and select a colour. Multiple elements can be adjusted at one time. 

Please note that stroke will also be adjusted on a Cake's Ribbon. So if you add a Ribbon to a Cake after adjusting the Cake's stroke colour, the Ribbon will not have the same Stroke adjustment and you will need to select the Cake and re-adjust the Stroke if you want the Ribbon to be included.