Okay,

here's short how-to about converting ANY other theme to act like Plastic Scheme. Things needed in this turorial:
ON DEVICE:
rootsh (install from extras)

1. you must have the theme installed onto the phone which you want to convert to be Plastic Scheme
2. locate the theme (type these into terminal):
That will output you directories of themes you have installed like this:
In this case we shall make the default NSeries theme to be Plastic Scheme. It's directory is called "alpha".

3. Create a directory for the scheme under /opt/plastic-schemes/, you can name the direcotry to what you want, name of the directory is shown as scheme name in Plastic Theme Setup. Type this into xterm:
4. Copy the contents of the theme to /opt/plastic-schemes/NSeries/ (or what ever you decided to name the directory in previous step), type these into terminal:
That's the easy part. Now we need to manually edit one file so that the theme will actually work as scheme. You can use what ever editor you want, I prefer leafpad since it can be easily be launched from terminal. Edit this file:
/opt/plastic-schemes/NSeries/gtk-2.0/gtkrc

If you're using leafpad too, then the command to edit this file would be:
When you have the gtkrc-file opened into some editor, locate these three lines (they are almost at the beginning of the file):
Replace those three lines with these:
When done, save the file and close the text editor and return to terminal. Next we need to search and replace all entries of "Nokia Sans" with "Droid Sans" in few files. Copy and paste these commands into terminal (you still must be gainrooted), these are both one-lines, the forum software seems to automaticly break the lines... but copy-paste works here:
Finally we need to remove some unwanted files. Type these into terminal:
That's it!

Now the NSeries -theme will be shown @ Plastic Theme Setup and you can customize it same way like all other schemes.

I personally use openssh-server and WinSCP -program where transferring and editing files is much easies when you can do all this with GUI.