Since version 3.0 you may customize PNotes.NET appearance by various themes. The default program's theme is Dark Gray:

Another themes can be downloaded here. After downloading just unpack the file into PNotes.NET program folder, restart the program and choose desired theme.

Here is the sample source which you can use as your theme's basis. It's actually .NET DLL project. After compiling change its extention to .pntheme.