Ande I've found the issue! First of all, in the custom CSS you used #centered-button
while the ID is set to center-button
Secondly, I think I've found a little issue: the custom ID is set to the button instead of the group... Could you temporarely add a second button to that group, then go to the settings of the group and set the custom id
for the group? After that, remove the tempory button.
Let me know if this helps 🙂