rock-star_79 , rule number one. Never Ever Ever "upgrade" with someone else's firmware. You will only end up killing your player. You will not gain any additional functions by "upgrading".
To mod the themes, firstly you will need to extract your existing firmware. The guide is
Here.
Always ensure that whatever you mod and flash into your player, is always your own original firmware file. This procedure will also create a dump.bin firmware file, which is your firmware backup, keep it safely.
Next, you'll need the S1res tool to mod the text/images. The guide is
Here.
You can find various theme packs that others have uploaded into the forum.
Just pay attention to those of 1.5" screen (which is your screen size).
However, not all the 1.5" screen themes would be able to suit yours, as some orientation of certain images are different from each other.
Finally after saving all these changes, you'll need Upgrade Tool to flash in the newly mod firmware. The guide is
Here.
Hope this helps.