set the background color for window (#2853)

* set the background color for window

* update changelog for #2853

v2/internal/frontend/desktop/linux/window.c

@@ -139,11 +139,31 @@ void SetWindowTransparency(GtkWidget *widget)
     }
 }
 
+static GtkCssProvider *windowCssProvider = NULL;
+
 void SetBackgroundColour(void *data)
 {
+    // set webview's background color
     RGBAOptions *options = (RGBAOptions *)data;
     GdkRGBA colour = {options->r / 255.0, options->g / 255.0, options->b / 255.0, options->a / 255.0};
     webkit_web_view_set_background_color(WEBKIT_WEB_VIEW(options->webview), &colour);
+
+    // set window's background color
+    GtkWidget *window = GTK_WIDGET(options->window);
+    gchar *str = g_strdup_printf("window {background-color: rgba(%d, %d, %d, %d);}", options->r, options->g, options->b, options->a);
+
+    if (windowCssProvider == NULL)
+    {
+        windowCssProvider = gtk_css_provider_new();
+        gtk_style_context_add_provider(
+            gtk_widget_get_style_context(window),
+            GTK_STYLE_PROVIDER(windowCssProvider),
+            GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+        g_object_unref(windowCssProvider);
+    }
+
+    gtk_css_provider_load_from_data(windowCssProvider, str, -1, NULL);
+    g_free(str);
 }
 
 static gboolean setTitle(gpointer data)

v2/internal/frontend/desktop/linux/window.go

@@ -271,6 +271,7 @@ func (w *Window) SetBackgroundColour(r uint8, g uint8, b uint8, a uint8) {
 		b:       C.uchar(b),
 		a:       C.uchar(a),
 		webview: w.webview,
+		window:  w.gtkWindow,
 	}
 	invokeOnMainThread(func() { C.SetBackgroundColour(unsafe.Pointer(&data)) })
 

v2/internal/frontend/desktop/linux/window.h

@@ -55,6 +55,7 @@ typedef struct RGBAOptions
     uint8_t b;
     uint8_t a;
     void *webview;
+    void *window;
 } RGBAOptions;
 
 typedef struct SetTitleArgs