There are several ways to do this (as has been discussed previously in the forum)…
Bear in mind CSS does not have a ‘float-centre’ property, so as pointed out already in this thread, the simplest way is just to have a full-width transparent FG at the top, then position your ‘visible element’ in the centre of that. You might need to add some cimple custom CSS to change the pointer events to ‘none’.
You can also us margin-left: auto;
Here’s another recent discussion of the same question for more details: Here’s how to center a floating group (new responsive engine) - Tips - Bubble Forum