Yes. Here's how:

1. Select the Colour tab in the left panel, to change the existing background colour of a Sticker.
2. Change the sticker's background colour by:
   1. Selecting one from the colour palette
   2. Selecting via the Eyedropper Colour Tool
   3. Specifying a custom colour or adjust the CMYK values
   4. You can also choose to have a transparent background by choosing the No-Colour option, i.e. the white circle with a redline in it.