Every form sits in a .form-island, on pages TYPO3 13 renders as well as in the form page type (/path/form) that the static pages load: the form posts there, and htmx swaps the island as a whole (see Form.html). While a request runs, htmx sets htmx-request on it and the overlay covers the form. Styled here and not in the theme build: the static pages around the island load the CSS of the TYPO3 10 export. The spinner takes the button colour of each theme.