converse.js/sass/inverse/_controlbox.scss

#conversejs.fullscreen {
    #controlbox {
        @include make-col-ready();
        @include media-breakpoint-up(md) {
            @include make-col(3);
        }
        @include media-breakpoint-up(xl) {
            @include make-col(2);
        }

        &.logged-out {
            @include make-col(12);
        }

        margin: 0;

        .controlbox-pane {
            border-radius: 0;
        }

        .flyout {
            border-radius: 0;
        }

        #converse-login-panel {
            border-radius: 0;
            .converse-form {
                padding: 3em 2em 3em;
            }
        }

        .toggle-register-login {
            line-height: $line-height-huge;
        }

        .brand-heading-container {
            @include make-col(12);
            text-align: center;
            .brand-heading {
                font-size: 150%;
                font-size: 600%;
                padding: 0.7em 0 0 0;
                opacity: 0.8;
                color: $blue;
            }
            .brand-subtitle {
                font-size: 90%;
                padding: 0.5em;
            }
            @media screen and (max-width: $mobile-portrait-length) {
                .brand-heading {
                    font-size: 400%;
                }
            }
        }

        &.logged-out {
            @include make-col(12);
            @include fade-in;
            width: 100%;
            .box-flyout {
                width: 100%;
            }
        }
        .box-flyout {
            border: 0;
            width: 100%;
            z-index: 1;
            background-color: $controlbox-head-color;

            .controlbox-head {
                display: none;
            }
        }

        #converse-register, #converse-login {
            @include make-col-ready();
            @include make-col(8);
            @include make-col-offset(2);

            @include media-breakpoint-up(sm) {
                @include make-col(8);
                @include make-col-offset(2);
            }
            @include media-breakpoint-up(md) {
                @include make-col(8);
                @include make-col-offset(2);
            }
            @include media-breakpoint-up(lg) {
                @include make-col(6);
                @include make-col-offset(3);
            }
            .title, .instructions {
                margin: 1em 0;
            }
            input[type=submit],
            input[type=button] {
                width: auto;
            }
        }
    }
}