gui/month-view: Use dimmed style class

Instead of manually setting the font color for the month view's
week header, the `dimmed` style class dims labels according to the
HIG while simultaneously accommodating high contrast users.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1437

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/624>

src/theme/style.css

@@ -33,7 +33,7 @@ calendar-view.header,
 label.header {
   font-size: 10pt;
   font-weight: bold;
-  color: rgb(from var(--window-fg-color) r g b / 55%);
+  opacity: var(--dim-opacity);
 }
 
 calendar-view.current,