Click the "HTML" button in the post editor toolbar. Tumblr switches the post editor to HTML mode. Scroll to the section of code that contains text or the object that you want to center.

Click "Open With." Click the HTML editor in the menu to load the code. Scroll down the page to where you want to place the Facebook button. Add the following code to include a Facebook button: ...

Image: Malwarebytes. However, when the user moves his mouse to close the popup, CSS code from that page will expand the popup and move the ad in the cursor's path, so any click on the close button ...