The date number, which appears in each date cell, is controlled by the two CSS instructions:

.month .cell.empty .day { color: #dddddd; }
.month .cell.full .day { color: #999999; }

Notice that .empty and .full selectors are stuck together with the .cell selector. Those classes appear on the same td element in the HTML.

The post number for dates which have posts are always going to be links. They just seem to use the link colors defined for the page:

a { color: #2452FF; }
a:visited { color: #142D8B; }
/* and probably a hover color too */

If you wanted to just change the calendar links, maybe something like this would work:

.month a { color: #rrggbb; }

The color of the date cells that have posts is controlled with:

.month .cell.full { color: #DDEEFF; }

Let us know if these hit the correct spots.