Increase datefield calendarpanel selected date text contrast

Make the text easier to read.

Change-Id: Ieab5b27cccd0694086f17877d6c6b4c37b225b02

WebContent/VAADIN/themes/valo/components/_datefield.scss

@@ -522,6 +522,7 @@
   color: valo-font-color($v-selection-color);
   @include valo-gradient($v-selection-color);
   border: none;
+  font-weight: max(600, $v-font-weight + 100);
 }