The CSS gradient is described here, but I have no idea how to select for these properties in JavaScript. I would rather not use jQuery for this if at all possible.
EDIT: Just doing the following doesn’t seem to work…
document.getElementById("selected-tab").style.background = "#860432";
document.getElementById("selected-tab").style.background = "-moz-linear-gradient(#b8042f, #860432)";
document.getElementById("selected-tab").style.background = "-o-linear-gradient(#b8042f, #860432)";
document.getElementById("selected-tab").style.background = "-webkit-gradient(linear, 0% 0%, 0% 100%, from(#b8042f), to(#860432))";
document.getElementById("selected-tab").style.background = "-webkit-linear-gradient(#b8042f, #860432)";
I’m not a JS expert, but my guess is that your settings overwrite each other, so you might want to create a css class selector for this like .gradientBackground and check out the link below:
Change an element's class with JavaScript