chore: migrate to GTK 4.20 CSS @media queries

Author: Dzheremi2

data/Chronograph.gresource.xml.in

@@ -17,7 +17,6 @@
     <file preprocess="xml-stripblanks">gtk/ui/widgets/wbw/LyricsWidget.ui</file>
     <file alias="shortcuts-dialog.ui" preprocess="xml-stripblanks">gtk/ui/shortcuts-dialog.ui</file>
     <file alias="style.css">gtk/css/style.css</file>
-    <file alias="style-dark.css">gtk/css/style-dark.css</file>
   </gresource>
   <!-- Resources (YAMLs, JSONs, etc.) -->
    <gresource prefix="@PREFIX@/resources/">

data/gtk/css/style-dark.css

@@ -41,4 +41,14 @@ flowboxchild:active {
   background: none;
   box-shadow: none;
   outline: none;
+}
+
+@media (prefers-color-scheme: dark) {
+  .rounded-frame {
+    border-top-right-radius: 12px;
+    border-top-left-radius: 12px;
+    border-bottom-right-radius: 12px;
+    border-bottom-left-radius: 12px;
+    background-color: #1d1d20;
+  }
 }