Shoutbox

In style files, the square brackets [ ] denote a condition. So element[id=atom(idUserTileVisibility)] would be the element with the condition that id=atom(idUserTileVisibility), that is:
IF id = atom(idUserTileVisibility) THEN give the element the following style

Following the same logic, element[id=atom(idUserTileVisibility)][visible=false] is saying:
IF id = atom(idUserTileVisibility) AND visible = false THEN give the element the following style

Now as you can see this is creating a problem, because you're not actually specifying that the element should not be visible, rather you're asking if it is not visible.

So what your existing code is doing is defining the style for when the element has its visibility set to true or false.
You are not actually telling it to be either visible or not, only what to look like if it is visible or if it is not visible.
In order to fix that you need to get rid of the [visible=true] and [visible=false] tags you added. So that it will apply the style codes regardless of if visible = true or visible = false, instead depending only on your ShowDP PlusSkin variable.

NOTE: I have not actually tried your code so I am not sure if it will work or not, but I don't think it will. You will most likely have to add your condition to the Definitions file instead (well you probably wouldn't have to, but it would be easier that way).