Hello Andrew Jayne,
You can change the background bar of each event by going to the Settings area in the actual Calendar Channel(s) that the particular widget is pulling from. Please see our documentation on changing the Channel Colour here. You can choose a darker colour, which will have more contrast with the white text.
If you would like more step-by-step instructions on how to do this, please feel free to send us a ticket at firstname.lastname@example.org.