Neon Green (#39FF14)

/* Background */
.element {
    background-color: #39FF14;
}

/* Text */
.element {
    color: #39FF14;
}

/* Border */
.element {
    border: 1px solid #39FF14;
}

/* Linear gradient to complementary */
.element {
    background: linear-gradient(
        to right,
        #39FF14,
        #FFFFFF
    );
}

/* Radial gradient */
.element {
    background: radial-gradient(
        circle,
        #39FF14,
        #FFFFFF
    );
}

// SCSS variable
$neon-green: #39FF14;

// With RGB channels (useful for rgba() usage)
$neon-green-r: 57;
$neon-green-g: 255;
$neon-green-b: 20;

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