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 Attributes

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.

Style Attributes

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: