How DO u make Custom Cell Backgrounds?

I'm trying to make custom cell backgrounds, but they all just produce that generic color. Putting html in the title or descrpition does nothing, so how do i do it? i'm stumped

[Velocd]

<style type="text/css">
.custombg {
  background-image: url(gradient.gif);
  background-color: black;
  background-position: top left;
  background-repeat: repeat-x;
}
</style>

<td class="custombg"></td>

Like this?