Midnight Blue (#191970)

/* Background */
.element {
    background-color: #191970;
}

/* Text */
.element {
    color: #191970;
}

/* Border */
.element {
    border: 1px solid #191970;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #191970,
        #C7C719
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #191970,
        #C7C719
    );
}

// SCSS variable
$midnight-blue: #191970;

// With RGB channels (useful for rgba() usage)
$midnight-blue-r: 25;
$midnight-blue-g: 25;
$midnight-blue-b: 112;

// Usage
.element {
    background-color: $midnight-blue;
    color: rgba($midnight-blue-r, $midnight-blue-g, $midnight-blue-b, 0.8);
}