I had this problem with Safari, only on my iPhone, but it showed up correctly in Firefox (using user agent switcher). If you’re ok with not having a gradient background, you can add this to “Custom CSS” in Graphene Neo Options under the “Display” tab (but change the color of course):
.return-top { background:#8b0000; }
This just replaces what the theme does with the linear gradient which is:
.return-top{background:linear-gradient(#8b0000, #8b0000)}