Fix MediterraneanDarkest theme

The transparent background has already been fixed for the MediterraneanNight theme at https://github.com/rbrito/pkg-mediterranean-gtk-themes/commit/5b1828292338d525e29b6281e40b08067feeef6e but not for the 	MediterraneanDarkest one.

The calendar font color has been wrong all the time, I guess. I changed it to @fg_color. I also tried @text_color but it's inconsistent with the other theme colors in the indicator bar/calendar app. Also the darker background in the calendar days doesn't suit the @fg_color well.