Theme Creation
Overview
To create custom themes, you will be required to learn or know about basic graphics creating & editing, and XML file editing.
Read ALL the documentation in the theme toolkit.
Example of the work involved
Creating and editing images to specific specifications and resolutions
Editing an XML file
Using Command line with the compiler to create the theme file executable
Read up on forums, and articles
Is There Easier Way?
Yes, there are some software that simplifies the creation of themes.
Check out this site - http://bestps3themes.com/create-ps3-themes-tutorial/
Creation Suggestions
Backgrounds
Bright / Light backgrounds with white icons are VERY HARD TO SEE. Consider using darker background or manipulate a bright image along the XMB bar.
Make both SD & HD
In this day and age, SD (480) seems like a dinosaur, but sersiouly consider making theme compatible with SD & HD tv's.
Pitfalls
No "Dynamic" Themes
Dynamic themes came out after the PS3 launched, and Sony never gave the ability for the community to create them.
Custom Icons
Sony never updated the theme creation (kits) after additional icons were added to the PS3 & PSP. If you create custom a custom icon set, you still see some "default" white icons on the XMB. Not sure if you can hack the XML file to include the additional icon informaiton.
Software
Here's some free software that can assasit you with creating themes.
Image Creation & Editing
GIMP (Windows, macOS) - http://www.gimp.org/
Paint.NET (Windows) - http://www.getpaint.net/index.html
XML Editing
Visual Studio Code - https://code.visualstudio.com/
Notepad ++ - https://notepad-plus-plus.org/downloads/