Zotero styles are written in an XML language called CSL (Citation Style Language) which is also used by other citation managers like Mendeley or Papers. To modify a style, there are 2 available tools:
You will maybe prefer the online editor if you are not very comfortable with tags, but the previsualization works with a limited number of item types.
You can request a style if you do not want to write it yourself. You should only request an existing style, the style of a journal for instance, and provide a link to online documentation.
Or you can create it yourself (if you have the time and the motivation). First, find a style close to the style you wish to create (Search by Example on the CSL Visual Editor) and make the necessary modifications.
If this style is an "official" style, you can have it validated, and it will be available for other users on the style repository. There is first an automatic validation to check the mistakes, then the file is validated by a human volunteer. The best way to have this done is to create a pull request on GitHub.