style: Harmonize the mini calendar's week numbers with GNOME Shell

Make the week numbers a bit clearer to look at in the mini calendar,
differentiating them from day numbers so it's easier to tell them apart.

The style is loosely inspired from GNOME Shell's calendar widget,
but retains GNOME Calendar's more compact layout.

Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/378
Related: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/934

src/theme/style-hc.css

@@ -3,3 +3,8 @@
 .sources-button > button:checked {
     box-shadow: inset 1px 0 @borders, inset -1px 0 @borders;
 }
+datechooser .weeknum{
+    color: @theme_fg_color;
+    background-color: alpha(@theme_fg_color, 0.05);
+    border: 1px solid alpha(currentColor, 0.5);
+}

src/theme/style.css

@@ -174,8 +174,16 @@ datechooser navigator button {
   padding: 0;
 }
 
-/* day names and week numbers*/
-datechooser .weeknum,
+/* week numbers, with a style resembling GNOME Shell's */
+datechooser .weeknum{
+    font-size: smaller;
+    margin: 6px;
+    border-radius: 3px;
+    background-color: alpha(@theme_fg_color, 0.1);
+    color: @insensitive_fg_color;
+}
+
+/* day names */
 datechooser .weekday {
   color: @insensitive_fg_color;
   font-size: smaller;