All the elements except Text and Straight Line have border width and border color properties. You can change the border width or border color by opening their properties window. Double click on an element(or select the element, right click on it and choose properties) to bring up the properties window. Now in the Border section - select a new border color, border width or border style .
For line elements, you can open up its properties window and change line color, line width or line style.