woocommerce/packages/js/components/src/experimental-select-control/combo-box.scss

.woocommerce-experimental-select-control__combox-box {
    flex-grow: 1;
    display: flex;
    align-items: center;

    input {
        width: 100%;
        padding: $gap-smaller 0;
    }
}

.woocommerce-experimental-select-control__combox-box-icon {
    padding-left: 10px;
    margin-right: -2px;
}