Adding a multicolored border to bottom of header

Hopefully a super simple solution but I can't seem to find what I'm looking for.

Our brand regularly uses black or white backgrounds with red-white-red stripes to separate sections. I want to add this red-white-red horizontal line to the bottom of our site header and top of our site footer. I've made something that looks right but doesn't function the way I want it to using a separate section.

Any way I can create this using CSS?

Can you share link to page? I think we can use CSS pseudo to achieve this

Sorry to reply so late!

The site is up at
https://primrose-dandelion-zjlf.squarespace.com/

That just gives me a 5px red line?

The 2nd line of code doesn't seem to do anything. If I erase everything after the 2nd curly brace nothing changes. 

This is what I'm seeing.

Perhaps this is an issue of what you see in Preview is different from what we are seeing as an outside viewer.

Try looking at your site in private browsing. How does it look?

Let us know how it goes.

Oh you're right! I opened it in a private browser and it works perfectly. Thanks so much!

Sorry @tuanphan, it's not quite what I need yet.

I need the lines under the header to be RED | WHITE | RED but with the code you wrote there is no opaque white, it's transparent:

Any ideas?

11 hours ago, pipedreamdesign said:

Sorry @tuanphan, it's not quite what I need yet.

I need the lines under the header to be RED | WHITE | RED but with the code you wrote there is no opaque white, it's transparent:

Any ideas?

Add this CSS under code I sent

header#header:before {
    content: "";
    position: absolute;
    bottom: -13px;
    left: 0;
    width: 100%;
    height: 7px;
    background-color: #fff;
}

14 hours ago, tuanphan said:

Add this CSS under code I sent

header#header:before {
    content: "";
    position: absolute;
    bottom: -13px;
    left: 0;
    width: 100%;
    height: 7px;
    background-color: #fff;
}

I adjusted the -13 to -14 and changed #fff to white, but otherwise absolutely nailed it! You're a saint @tuanphan!