You can add or edit the border of an element, to change its color of its size.
To do so, select the element of your choice to display the tool panel, then click on the "More options" button to display all the settings.
In the "Background" part, click on "Border" to see the border settings.
A new window appears, allowing you to define the border size, in pixels.
If you want to apply the same size to all borders (sides, top, bottom), click on the locker on the right.
To apply rounded corners, select the corners of your choice, then fill in the value or the corner. When a corner is selected, it appears in green on the plan.
As for the "Color" tab, it allows you to choose or edit the border color.