On XFCE4 panel, the clock plugins background is based on whatever the background of the panel is at the left side of the panel. Whatever the panel background is is what will show behind the clock.
It does not have a configuration for changing the colors.
However, you can download and install "Orage" calendar:
- Code:
sudo apt-get install orage
This plugin DOES have the options to change foreground and background colors.
After installation, right click the panel. Select "Add new items" and scroll down to Orage. Add.
Right click on the new Orage Clock on your panel and choose "Properties."
From there, you can customize to fit your needs.