node: Remove transparency in dark mode, improve contrast to graphview in light mode

src/style.css

@@ -35,8 +35,10 @@
     color: black;
 }
 
-graphview {
-    background-color: @view_bg_color;
+node {
+    /* Compared to the default card color, this is not transparent in dark-mode
+       and provides a better contrast to the background in light mode */
+	background-color: @headerbar_bg_color;
 }
 
 node label.heading {

src/ui/graph/graph_view.rs

@@ -128,6 +128,8 @@ mod imp {
         fn constructed(&self) {
             self.parent_constructed();
 
+            self.obj().add_css_class("view");
+
             self.obj().set_overflow(gtk::Overflow::Hidden);
 
             self.setup_node_dragging();