una/studio/template/css/options.css

div.bx-stg-cells {
    position: relative;
    display: flex;
}
div.bx-stg-cells div.bx-sc-cell {
    position: relative;
    flex: 1 1 auto;
}
div.bx-stg-cells div.bx-sc-cell .bx-btn {
    display: inline-flex; /* keeps the 32px Studio button's centred label */
    float: none;
    vertical-align: middle;
}

/*-- Manage --*/
div.bx-stg-manage div.bx-sc-cell.bx-sc-right {
    text-align: right;
}

/*-- Mixes --*/
div.bx-stg-mixes div.bx-sc-cell.bx-sc-select {
    width: 50%;
}
div.bx-stg-mixes div.bx-sc-cell.bx-sc-new {
    width: 50%;

    text-align: right;
}
div.bx-stg-mixes div.bx-sc-cell select {
    width: auto;
}