I've been using a custom GTK+ RC style for the notes plugin since the version 1.4.0, right now it is at version 1.7.2. I have been playing with GTK+ theming again these last two hours, and I've get custom scrollbars, a gradient for the custom-made “title bar”, and better colours for the notebook to get the current tab stand out from the crowd.
While experimenting on a test-case code I found out a better way to parse a gtkrc file in the program. The first time I was fighting with the existing gtk_rc related functions, I gave up on a solution I partially dislike that is to include a line to the custom gtkrc file within ~/.gtkrc-2.0.
Today I understood how gtk_rc_parse(filename) behaves. You have to call this function at the beginning of the program before building any widgets, it will work even if the file doesn't exist yet. Next, while the program is running, you can modify the file, create it, delete it, truncate it, whatever, and call gtk_rc_reparse_all() to get the style refreshed in the GUI. It's hard to believe that such easy things are sometimes a PITA :-)
Be prepared for a 1.7.3 notes plugin with nicer colours.