Fix #1260.

Setters without an activator should binding with `Style` priority, not `StyleTrigger`.

Fixes #1260

src/Avalonia.Styling/Styling/Setter.cs

@@ -174,7 +174,7 @@ namespace Avalonia.Styling
             }
             else
             {
-                return sourceInstance.WithPriority(BindingPriority.StyleTrigger);
+                return sourceInstance.WithPriority(BindingPriority.Style);
             }
         }
     }