Update test over user css over widget css to show important widget css can be overridden by users too

sandbox/darren/just_a_box.css

@@ -2,5 +2,5 @@
     height: 50%;
     width: 50%;
     align: center middle;
-    background: green;
+    background: blue;
 }

tests/css/test_stylesheet.py

@@ -108,7 +108,7 @@ def test_stylesheet_apply_user_css_over_widget_css():
     user_css = ".a {color: red; tint: yellow;}"
 
     class MyWidget(Widget):
-        CSS = ".a {color: blue; background: lime;}"
+        CSS = ".a {color: blue !important; background: lime;}"
 
     node = MyWidget()
     node.add_class("a")