Put this in css_additional.css
style manager> style name> edit templates> css templates> css_additional.css

This is the basic.
Work on user drop down and login drop down.
But not tested with other drop down forms it may affected on them
/* move to bottom */
#main-navbar, #main-navbar-wrapper {
position:absolute;bottom:0px;left:0px;
}
/* parent holder */
html{position:relative}
/* give space for the login bar */
#footer-copyright {
margin-bottom: 70px!important;
}
/* made the user drop down - drop upward */
.b-menu__dropdown-submenu {
top: auto!important;
bottom:45px!important;
}