Only set background color on tree if "linesVisible" is false

On macOS setting "linesVisible" to true creates an zebra styled pattern
on the tree. If we now set the background color via CSS
on this tree that pattern would be gone. Setting the background color
via CSS on a tree that has "linesVisible" to false does not do any harm.

bundles/org.eclipse.ui.themes/css/e4_preview_mac.css

@@ -103,7 +103,6 @@ CTabFolder Canvas {
 }
 
 .View Composite,
-.View Composite Tree,
 .View Composite Label,
 .View ToolBar,
 .View Group,
@@ -145,6 +144,11 @@ CTabFolder Canvas {
 	swt-tabBackground-color: #ffffff;
 }
 
+
+.View Composite Tree[swt-lines-visible=false]{
+	background-color: #f8f8f8;
+}
+
 .View Composite PrependingAsteriskFilteredTree,
 .View PrependingAsteriskFilteredTree Text,
 .View Group Text,