When using a light theme in XFCE the clock is unreadable because it is displayed as light grey text on a light grey background.
Does anyone know if there is a way to alter these colours without switching to a totally different theme?
You might, also, go into the panel Preferences and Add the Orage Clock.
The Orage Clock is very customizable ... changing the Font, Size, Color and basic Orage-Clock layout
If you like the Orage Clock, then, you should 'remove' the default clock from the panel.