Check out this article for inspiration. I took some pointers from this SO post, too.
Basically, you need to style all the selectors. I played with this a bit, and everything works as expected. Here are the selectors, play with them to figure things out.
progress {
/* style rules */
}
progress::-webkit-progress-bar {
/* style rules */
}
progress::-webkit-progress-value {
/* style rules */
}
progress::-moz-progress-bar {
/* style rules */
}
Here’s what I got using the SO values…

Looks pretty clunky, but helped me figure out what selectors do what.
Hope that helps!