Widget in center

[admin]

Make sure that Mosaic is updated to the latest version. Add this Custom CSS to Mosaic / Settings & themes. This rule only applies to the Main screen.

Code:

@media (max-width: 640px) {
  .route_main .w-constrain {
    width: 330px !important;
  }
}

Adjust widths as needed. 640px is the maximum screen width when the rule is applied. 330px is the resulting widget container width (3 widget units).