All the code you can find here or libraries (.jar files) you can download are free. You can use it or extend/modify it as you wish. The only request is to add reference to me in the used code snippets.
Table of Content
If it's your first EditorKit customizing project or miss some basic concepts you can start from the EditorKit concepts and tutorial to understand how it's organized and works. If you find some missing explanations or concepts or need more detailed description you can send me your suggestions and I'll try to add more information to the tutorial.