gtk3-demo: Fix some css warnings

Update the font: syntax to follow css.

demos/gtk-demo/css_accordion.css

@@ -4,7 +4,7 @@
     transition-property: color, background-color, border-color, background-image, padding, border-width;
     transition-duration: 1s;
 
-    font: Cantarell 20px;
+    font: 20px Cantarell;
 }
 
 window {

demos/gtk-demo/cssview.css

 /* Make the text editor has a nice style */
 .view {
   color: #2e3436;
-  font: Monospace;
+  font-family: Monospace;
   background-color: alpha(white, 0.30);
 }