Check out the templates under modules\PostCalendar\pntemplates\default\views\calendarblock they often use the textcolor and bgcolors specified at the start of your theme... You can either change those values in your theme's theme.php or hardcode the values into the PC template (when making changes to PC templates dump the template cache - in PC admin menu - to see changes)...
You loose a bit of flexibility if you have a site using multiple themes... The hardcoded value might not look right from one theme to the next. But, IMO, most *professional* websites stick with one general layout (with minor variances) to present a more cohesive, unified look - instead of letting users switch from one completely different look to another...