Need a border line underneath header

Site URL: https://www.interiorsbyhonor.com/

Hi,

I require a navy blue line to seperate header from content below. It needs to stay the same across all pages.

I have attached an example how it should look like. I am unsure how to code to get the line at this part of the website.

Add to Home > Design > Custom CSS

.header-announcement-bar-wrapper:after {
    content: "";
    display: block;
    background: black;
    width: 100%;
    height: 1px;
}

This works as well, heck of a lot simpler lines of code than I thought it would take. 

header#header {
    border-bottom: 2px solid blue;
}

6 hours ago, derricksrandomviews said:

This works as well, heck of a lot simpler lines of code than I thought it would take. 

header#header {
    border-bottom: 2px solid blue;
}

this is not working for me? thoughts

Remove the double close on the last line not supposed to be two of these }