Line 1072 of the assets/css/main.css file. It also uses an image for the background. Or it may be the main_wrapper background, which is set on line 1690 of the same file.
I don't like the html5up templates. They are way too complex and use some kind of proprietary javascript controls.
[ed. note: sottwell last edited this post 8 years, 9 months ago.]
Enable developer tools in your browser and inspect those parts. The inspector tool will show you where it's getting its styling from.
Try clearing your browser cache. Sometimes background images are stuck in the browser cache.