Well

1 -> Create a folder with the name of your theme
2 -> Create a css file if you want
3 -> Create a pictures.ini file and put the pictures in the folder too
4 -> Add fonts needed by the CSS in the Themes/YourTheme/Fonts folder

=== Detailed ===

1 -> No explanation needed. The folder you'll create will contain everything about your theme.
If something is missing, PO will always look in the Default folder for that thing, so worry not.

2 -> The default.css file manages all the "styling" of the fonts, backgrounds, ...
It's advised to copy the one in the Default folder, to your folder, and then edit it and
test your edits using Reload Stylesheet.

Note: The CSS file uses some pictures directly (like mainchat background, scrolling pokeball for EVs)
So if you want to change the main chat background, you have to create the picture in your theme
folder and then in the default.css change the location where it gets the picture.


3 -> Pictures. You can override the default pictures. For that put some pictures in your theme
folder. Then create a pictures.ini file. Look at the pictures.ini file that's in the Default folder.

Then add the pictures paths correctly. If you use the same "key" as in the default pictures.ini file, 
then the path you specify will be used instead of the default pictures.ini file.

_________

For any question go on the forums. Or look at the created themes, apart from Default.