Padding was removed from the Joomla admin screen in this commit - https://github.com/civicrm/civicrm-core/commit/
6ada1cee014a8bd99e4778864c32c0c140c7a92e - which removed all padding on the upgrade screen. Fix to add padding to that screen.
padding: 0;
border-top: 1px solid #787878;
}
+body.admin.com_civicrm.task-civicrmupgrade .container-fluid.container-main {
+ padding: 0 20px 20px;
+}
body.admin.com_civicrm #crm-nav-menu-container {
padding-bottom: 0 !important;
}