Most of the themes uploaded to site or posted in the forum have screen sized preview images and animated gif's.
To test drive how the theme will look on your player, right click on the preview image and select "Save Picture As.. " and save to your PC. Then you can load it to your player and view it with your picture viewer. This way you can see how the theme will look on your small screen.
Here are a few you can try
http://www.mympxplayer.org/viewtopic.php?t=1409
http://www.mympxplayer.org/viewtopic.php?t=743
http://www.mympxplayer.org/viewtopic.php?t=1472
http://www.mympxplayer.org/viewtopic.php?t=651