Fix fallback image if dark theme is used

When using a dark theme, the RSS icon will appear in (nearly) white on a
white background.  Let's fix the icon colour to black to always get the
same fallback image.

src/qml/ImageWithFallback.qml

@@ -67,6 +67,8 @@ Item {
             Kirigami.Icon {
                 anchors.fill: parent
                 source: "rss"
+                isMask: true
+                color: "black"
             }
         }
     }