fix(dialogs.scss): Separate dialog text color from border color

Previously, we were using the `color` property to determine the color of the border around `<dialog
/>` elements. The issue with this is that it requires the text color to match the border color. I've
fixed this by moving the border color declarations into the `box-shadow` property.

scss/elements/dialogs.scss

@@ -11,8 +11,7 @@
   }
 
   &.is-rounded {
-    color: $base-color;
     border: none;
-    box-shadow: 4px 0, -4px 0, 0 4px, 0 -4px;
+    box-shadow: 4px 0 $base-color, -4px 0 $base-color, 0 4px $base-color, 0 -4px $base-color;
   }
 }