apprt/gtk: move some static CSS to the style.css file (#4011)

src/apprt/gtk/App.zig

@@ -972,9 +972,6 @@ fn loadRuntimeCss(
     const headerbar_foreground = config.@"window-titlebar-foreground" orelse config.foreground;
 
     try writer.print(
-        \\window.without-window-decoration-and-with-titlebar {{
-        \\  border-radius: 0 0;
-        \\}}
         \\widget.unfocused-split {{
         \\ opacity: {d:.2};
         \\ background-color: rgb({d},{d},{d});

src/apprt/gtk/style.css

@@ -33,6 +33,10 @@ label.size-overlay.hidden {
   opacity: 0;
 }
 
+window.without-window-decoration-and-with-titlebar {
+  border-radius: 0 0;
+}
+
 .transparent {
   background-color: transparent;
 }