joomla/media/plg_system_wf_responsify/css/responsive.min.css
Disabling it, everything is OK:

The problematic part is height auto here:
.wf-responsive-iframe, .wf-responsive:not(table) {
max-width: 100% !important;
height: auto;
}
Seems like I can't have button colours AND modal box. This is killing meseems like Gantry Bootstrap: joomla/media/gantry5/engines/nucleus/css-compiled/bootstrap5.css does not have CSS information for buttons (in this case btin-success, btn-info)
Just for checking:@Christine
For the template I select the "default" in STYLES. The frontpage of my website is changed but when I type the phocadownload full address in my browser bar (namesite/joomla/phocadownload/methodology for example), I get the Gantry template!
(though Cassiopeia is default on ALL pages!