You can use the Rectangle tool to make a background and fill this with a color to form a backdrop to your store banner.
Select the Rectangle tool from the toolbar and click and drag on the banner to draw a rectangle that matches the dimensions of the banner. If necessary, you can fine tune the size and position of the rectangle by editing the object's properties in the Options bar. You can see the settings in the accompanying image.
You can set a color from the palette below the window or you can go to Object > Fill and Stroke and mix a color. You can also set a border color if you like, though I've chosen not to.