Ghost/core/client/app/styles/components/splitbuttons.css

/* Splitbuttons
/* ---------------------------------------------------------- */

.splitbtn {
    position: relative;
    display: inline-block;
    vertical-align: middle;
    /* Flatten out the right side */
    /* Flatten out the left side */
}

.splitbtn .btn {
    position: relative;
    float: left;
    /* Prevent double border between buttons */
    /* Make sure the hovered element is always on
    // top so overlap from .btn + btn. invisible */
}

.splitbtn .btn + .btn {
    margin-left: -1px;
}

.splitbtn .btn:hover,
.splitbtn .btn:focus,
.splitbtn .btn:active,
.splitbtn .btn.active {
    z-index: 2;
}

.splitbtn .btn:first-child {
    margin-left: 0;
}

.splitbtn .btn:first-child:not(:last-child):not(.dropdown-toggle) {
    border-top-right-radius: 0;
    border-bottom-right-radius: 0;
}

.splitbtn .dropdown-toggle {
    padding-right: 12px;
    padding-left: 12px;
    border-top-left-radius: 0;
    border-bottom-left-radius: 0;
    /* This is the additional dropdown arrow, to the right of the button. */
}

.splitbtn .dropdown-toggle.btn-sm {
    padding-right: 10px;
    padding-left: 10px;
}

.splitbtn .dropdown-toggle.btn-lg {
    padding-right: 16px;
    padding-left: 16px;
}

.splitbtn .dropdown-toggle .options {
    color: #fff;
    text-align: center;
}