Fix a certain div in ion-content

In the ion-content I want to have a fixed line of text, which is always horizontally and vertically aligned to the center of the ion-content and overlays all the other stuff in it.
Also, when one is scrolling (up or down) the line of text should stay in the middle!

I tried this directive, but it does not seem to work with ion-content.

What do I have to change?