Dark Midnight Blue (#003366)

/* Background */
.element {
    background-color: #003366;
}

/* Text */
.element {
    color: #003366;
}

/* Border */
.element {
    border: 1px solid #003366;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #003366,
        #CC6600
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #003366,
        #CC6600
    );
}

// SCSS variable
$dark-midnight-blue: #003366;

// With RGB channels (useful for rgba() usage)
$dark-midnight-blue-r: 0;
$dark-midnight-blue-g: 51;
$dark-midnight-blue-b: 102;

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