The filled rectangle shows the current fill colour. To change it, click the Fill Colour box.

For a transparent area, click the 'None' tick box.
For a colour, click on the Fill Colour ... button.

A Colour Picker box opens.Choose a style by clicking on a pre-defined colour, or click the Custom Colours button to choose your own.

Click OK to apply the style. You can then select another option from the Style dialog box.if required.