Replace style-*.css with media queries

Libadwaita 1.9 deprecates using style-dark.css, style-hc-dark.css and
style-hc.css. This moves these styles into style.css and conditionally
loads them using media queries.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/745>

src/theme/style-dark.css

@@ -1,8 +0,0 @@
-/*
- * Search
- */
-
-searchbutton > popover listview > row:selected {
-  background-color: rgb(from var(--accent-bg-color) r g b / 20%);
-}
-

src/theme/style-hc.css

@@ -1,9 +0,0 @@
-datechooser .weeknum {
-  --border-width: 1px;
-  --base-color: var(--window-fg-color);
-}
-
-calendar-view.month-view {
-  --raised-shade-color: var(--border-color);
-  --raised-shade-color-base: transparent;
-}

src/theme/style.css

@@ -142,6 +142,13 @@ datechooser .weeknum {
   background-color: rgb(from var(--base-color) r g b / 5%);
   color: var(--base-color);
 }
+/* high contrast */
+@media (prefers-contrast: more) {
+  datechooser .weeknum {
+    --border-width: 1px;
+    --base-color: var(--window-fg-color);
+  }
+}
 
 /* day names */
 datechooser .weekday {
@@ -377,6 +384,13 @@ calendar-view.month-view {
     inset 0 -1px var(--raised-shade-color, var(--raised-shade-color-base)),
     inset 0 -2px 4px var(--raised-shade-color-base);
 }
+/* high contrast */
+@media (prefers-contrast: more) {
+  calendar-view.month-view {
+    --raised-shade-color: var(--border-color);
+    --raised-shade-color-base: transparent;
+  }
+}
 
 calendar-view.month-view > box.month-header {
   padding-bottom: 6px;
@@ -443,6 +457,12 @@ searchbutton > popover > arrow {
 searchbutton > popover listview > row:selected {
   background-color: rgb(from var(--accent-bg-color) r g b / 11%);
 }
+/* dark mode */
+@media (prefers-color-scheme: dark) {
+  searchbutton > popover listview > row:selected {
+    background-color: rgb(from var(--accent-bg-color) r g b / 20%);
+  }
+}
 
 /*
  * Month selector

src/theme/theme.gresource.xml

@@ -3,8 +3,5 @@
   <gresource prefix="/org/gnome/calendar">
     <file>events.css</file>
     <file>style.css</file>
-    <file>style-dark.css</file>
-    <file>style-hc.css</file>
-    <file>style-hc-dark.css</file>
   </gresource>
 </gresources>