css: Use box shadow for month cells

This is probably a better approach for getting month separators,
because box shadows don't affect the layout, unlike borders. Since
GtkStyleContext is deprecated, it makes more sense to use properties
that don't have any side effects with physical size.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/601>

src/theme/style.css

@@ -234,10 +234,12 @@ button.week-header-expander:checked image {
 
 monthcell {
   --bg-color: rgb(from var(--window-fg-color) r g b / 0);
-  --separator-color: rgb(from var(--window-fg-color) r g b / calc(var(--border-opacity) + 12.5%));
+  --separator-color: rgb(from var(--border-color) r g b / calc(alpha + 7.5%));
+
+  box-shadow:
+    0 -1px var(--border-color),
+    var(--offset) 0 var(--border-color);
 
-  border: solid 1px rgb(from var(--border-color) r g b / calc(var(--border-opacity) / 1.5));
-  border-width: 1px 0 0 1px;
   transition: background-color 200ms;
   background-color: var(--bg-color);
 }
@@ -288,24 +290,25 @@ monthcell button {
 }
 
 /* Separators */
+monthcell:dir(ltr) {
+  --offset: -1px;
+}
+
+monthcell:dir(rtl) {
+  --offset: 1px;
+}
+
 monthcell.separator-top {
-  border-top: 1px solid var(--separator-color);
+  box-shadow:
+    0 -1px rgb(from var(--separator-color) r g b / calc(alpha * 2)),
+    var(--offset) 0 var(--border-color);
 }
 
-monthcell.separator-side:dir(ltr) {
-  border-left: 1px solid var(--separator-color);
-}
-
-monthcell.separator-side:dir(rtl) {
-  border-right: 1px solid var(--separator-color);
-}
-
-monthcell:nth-child(7n + 1):dir(ltr) {
-  border-left-width: 0;
-}
-
-monthcell:nth-child(7n + 1):dir(rtl) {
-  border-right-width: 0;
+monthcell.separator-top.separator-side {
+  box-shadow:
+    0 -1px var(--separator-color),
+    var(--offset) 0 var(--separator-color),
+    var(--offset) -1px var(--separator-color);
 }
 
 /*