You can find the Z-Index option in different parts of the elements. In this specific case the Bar z-index changed not the Content drop-down:
For the button, I clicked on the Customize tab of the Content Dropdown element and used the Element CSS feature to add the code below:
For more information about the
FLEX display mode and the code I used to center the button please read this article.
1- I have found the proper CSS code selector using the Chrome browser Developer Toolbar:
2- For the CSS code itself, I suggest that you get started with this tutorial: