How can I change the color of the horizontal rule in my header?

Currently it's set to black and would need to input some code to change it, thank you. 

This is the needed code to change the color: 

.form-header-group {

    border-color: red;

}

Replace the "red" value with the color that you want, this guide will also help you to inject the code to your form: How-to-Inject-Custom-CSS-Codes

Hope this helps.