The Inspector on the right lets you adjust the settings for the current layer, or the options for the current tool. When you have a layer selected you will see that the inspector is divided into sections.
General layer styles are in the top section; the position, size, opacity, blending and (depending on the layer type) a few special options, like corner radius for a rectangle or the number of points for a polygon shape.
Border and Fill attributes each have its own section.
To add a new Fill or Border, just click the + icon in the section title:
When adding a new Fill or Border, you’ll get a popover where you can choose the color:
You can turn each Fill or Border off by unchecking the checkbox on its left. When one or more Fills / Borders are off, you can delete them by clicking on the Trash Can icon that appears on the section title:
To change the options for a Fill or a Border, click the Gear icon: