On the Poxnora website you can change the theme/background in the top left corner. I was using the KF theme for years, I just now finally changed to IS.
Funny enough, I've been using the KF theme for years as well. Changed it about a week ago to the Poxnora theme.
The FS one has nice colors. From old ones before the revamp, the IS one was amazing (the blue one with inquisitors), loved it. Good times.
The Pox Nora one -- has a bunch of different images on it. Used Kanen for a while, but it's mainly a bunch of weird Kanen musculature. Would use FW, except it has a damn witch taking up half the screen.
My god you can change those.... I've been traumatized because the k'thir fairy only shows half her face and has been staring at me with one eye since i started playing again.
I always used the Tortun theme for some reason. The colours look nice, and not out of place. Was better when Tortun Captain was used, though.
Themes should have suiting backgrounds (landscape for example) for each faction, instead of close-up displays of a unit on each side (I dont like it tbh). That, or backgrounds displaying several units (such as the Poxnora Theme) for a certain faction.