[Theme: Onetone] Change background to solid color only on mobile

I’m having an issue where the background image used for the parallax on the desktop site repeats itself vertically all the way down my one-page website when I’m on my iPhone. Instead of trying to fix that, I’d rather go with the simple fix of having a solid color as the background, but only on mobile devices. I got a custom CSS started, but I’m not very proficient in programming and it doesn’t work.
Where am I going wrong?

@media only screen and (max-width: 1100px) {
background-color: #ffffff !important;
}

Thanks,

Michael