css: Adjust week number borders

We should generally avoid setting padding without subtracting with
the border width. On high contrast style, week numbers receive a
1px border width without subtracting it from the padding. This
causes a layout shift and makes the sidebar wider.

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

src/theme/style-hc.css

@@ -1,7 +1,8 @@
 datechooser .weeknum {
+  --border-width: 1px;
+
   color: var(--window-fg-color);
   background-color: rgb(from var(--window-fg-color) r g b / 5%);
-  border: 1px solid rgb(from currentColor r g b / 50%);
 }
 
 calendar-view.month-view {

src/theme/style.css

@@ -91,10 +91,16 @@ datechooser navigator button {
 
 /* week numbers, with a style resembling GNOME Shell's */
 datechooser .weeknum {
+  --padding: 4px;
+
+  --_border-width: var(--border-width, 0px);
+  --_padding: calc(var(--padding) - var(--_border-width));
+
   font-size: smaller;
   font-weight: bold;
   margin: 6px;
-  padding: 4px;
+  padding: var(--_padding);
+  border: var(--_border-width) solid rgb(from currentColor r g b / 50%);
   border-radius: 5px;
   background-color: rgb(from var(--insensitive-fg-color) r g b / 10%);
   color: var(--insensitive-fg-color);