CSS to stretch my header BG image

Hello,
I am working on a site https://www.rih20.com its a non-profit water day event. They have asked that the background behind the logo in the header stretch 100% of the page. I have tried everything I can think of. Anyone have any ideas how I can do this?

#header {
background: url(https://www.rih20.com/wp-content/themes/f8-lite/images/Top-Logo-Background.png);
background-repeat: repeat-x;
height:150px;
margin:0 0 0 0px;
padding:0;
width: 100%;
}
This is my css code for that section. The width is 100%….
Thanks in advance.
Joe