ah, this is mostly from my gtk theme, but now that i just remembered i did do a little bit of tinkering. i can't remember if there was anything else i needed to do, but i know that i needed to save this to .config/gtk-3.0/gtk.css. this gives the rounded edges, and corrects the foreground/background colours. i can't remember what it looked like originally but very few gtk themes actually matched well with the bar.

.xfce4-panel widget grid button.flat.toggle:checked
{
color: black;
border-radius: 10px;
}
.xfce4-panel widget grid button.flat.toggle:hover
{
color: black;
background-color: lightgrey;
border-radius: 10px;
}
.xfce4-panel .tasklist .toggle:checked
{
color: black;
border-radius: 10px;
}
.xfce4-panel .tasklist .toggle:hover
{
color: black;
background-color: lightgrey;
border-radius: 10px;
}