Hey Chelawonk,
this option has not been explicitly envisaged in Ikebana as a back-end setting, primarily because a centered button would obscure parts of the content, which is not good for the user experience. However, if you need to, you can center the scroll-to-top element by adding this line to the “Style: Add Custom CSS” tab of the Theme Options panel:
.scrolltop { right: 50% !important; }
Sincerely,
Andrii / Satori Studio