Please do post. It'd be nice to show more info than what's there with the default setup. I used RockBox years ago on my Clip and Clip+ and never even thought of looking for a better skin for the X3.
With the newer players showing album art which I'm sure is because of smartphones being able to play music it was odd going back to a player that only displayed text so I never even thought of skins. I've seen mention of the Walnut player which has no display at all. I had that sort of player 20 years ago back when SD cards, the full sized ones, were a big deal if you could get a 512Mb size. And even then with the number of 96k mp3 files made keeping track of what you were playing seemed daunting. I can't conceive of a 256Gb card and all those songs and no display on a player.
As far as themes, they have some really nice ones, that give more useful information and have a better layout than the stock theme.
All of the themes are here:
http://themes.rockbox.org/index.php?target=sansaclipplus
You just download & unzip the files onto the card, and use the "Settings - Theme Settings - Browse Theme Files" option in Rockbox to change between them.
I was able to find some really nice Base Skins and WPSs. I'm sooo glad I did. The ones I'm using now are cleaner, easier to read, cooler looking, etc than the 2 built-in ones. There's really simple themes, nerd overload themes, large text themes, alternate language themes, and even 'themed' themes (Pac-Man, Pokemon, Star Trek, Aliens, etc).
Check them out on that link - there's probably 75 different ones. After trying 15 of them, I narrowed it down to 2 in particular - Sweet and Balanced.
Finally, note that you can mix-and-match themes. In other words, you can change the whole theme (which includes the base "skin" and the WPS ie "While Playing Screen"). Or you can use the base "skin" from 1 theme, and the WPS from a different screen. The latter is what I did, because despite finding a base skin I liked, I didn't care for the WPS that came with that theme. Likewise, I liked the WPS that came with a different theme, but not the base skin. So I just took the parts of each theme that I liked, and ended up with the ultimate "franken-theme" setup.