You mean the “Back to top” button at the very end of the page? You could adjust that with CSS:

body .wp-block-button.is-style-outline > .wp-block-button__link, body .wp-block-button .wp-block-button__link.is-style-outline { border-width: 1px; }

You would have to put this in an individual CSS, e.g. per: https://wordpress.org/plugins/simple-custom-css/

Glad if I could help. You are welcome to set the topic to solved.