It is becouse this module with own css files ovveride default css classes for boostrap,

to fix it it is needed to ovveride it css file

To do that create files in path

themes/warehouse/modules/colissimo/views/css/colissimo.modal.css

with content

/* empty*/
.colissimo-front-widget-17.modal.in .modal-content {
    transform: scale(1);
    opacity: 1;
}
.colissimo-front-widget-17.modal.in  {
    opacity: 1 !important;
}
#checkout .modal-backdrop.fade.in {
    opacity: 1 !important;
}

Then clear prestashop and browser cache