unknown, I had the same problem. You need to download the following programs.
s1fwx
DOWNLOAD
MP3 Player Tools
DOWNLOAD
FirmwareModify
DOWNLOAD
You need to extract your firmware from your player using s1fwx as discribed in the Knowledge Base
HERE
Do not forget to repair it using repair.bat.
Then using FirmwareModify open your dump.bin file. At the bottom you will see an entry "Screen Contrast Adjusting" Set the slider to 20, then click Apply and OK to exit program.
Now you need to reload your firmware back to your player using the MP3 Upgrade Tool which is part of the MP3 Player Utilities 3.68 you downloaded.
You will find a guide on how to do this
HERE
Once your player reboots you screen should be ok.