styles.css

/* Set the border to a hard rectangular shape */
.dash-dropdown .Select-control {
    border: 2px solid rgb(37, 55, 77);
    border-radius: 4px;
}

/* Set the background color for the closed dropdown */
.dash-dropdown .Select-control {
    background-color: rgb(50, 70, 101);
}
.dash-dropdown.is-minimized .Select-value-label {
    color: rgb(235, 235, 235);
}
/* Set the text color */
.dash-dropdown .Select-value-label {
    color: rgb(235, 235, 235);

}

/* Set the background color for the open dropdown */
.dash-dropdown .Select-menu-outer {
    background-color: rgb(50, 70, 101);
}