multiple_on_overflow doesn't work for single digit days and fg highlighting

If the highlight is applied to the foreground, the days 1-9 can only show one color. At the moment the displayed color seems to be randomly chosen on startup (both with the calendar command and in interactive mode).
Since the overflow color only applies for 3 or more calendars on the same date and the priority has no effect in interactive mode/disables the two color display in the calendar command, the more important event may be overlooked.

This could be fixed by applying the overflow for two calendars on single digits, setting bg highlight where no fg character can be colored, or adding a leading zero (which would look most consistent in my opinion).

Reproduce the highlighting in both the calendar command and interactive mode using single-digit days with multiple calendars. Inspect the overflow and foreground/background highlighting paths; done when overlapping events on days 1–9 consistently expose the important colors without the reported random startup behavior.