You can also control opacity using the Alpha slider. The dropdown that currently reads "Radial" lets you pick between solid fill, linear gradient, radial gradient, bitmap fill, or no fill at all. When the gradient fill options are selected, a new color slider will appear that will allow you to determine the number of colors in your gradient, their shades, and their positions.
To switch back to default black and white, click the little black and white block below the fill marker. The slashed button next to it denotes "no fill" or "no stroke", and the button next to that will switch the colors for your stroke and fill.
To change between stroke and fill options, simply click on the appropriate button in the upper left corner.