An Anno-J style asset is nothing more than a CSS file. These files may be created and hosted by any third-party designer.
Rendering in AJ is typically achieved via drawing commands to HTML Canvas (although plugins may use the raw DOM if desired). Certain aspects of style fall under the control of CSS, for example, background images, boreders, colours and sizes. Not all CSS attributes are respected as certain attributes conflict with programmatically defined layout.
The first step, as a CSS designer, is to figure out what classes you will be creating styles for. This is typically service-specific, so you will need to consult the documentation for the service in question. Failing that, if you are using Firefox, just enable the Firebug plugin and look at the class names that are being streamed across the network.
Once a CSS has been created, an Anno-J administrator can make use of it by simply linking to it in the document <HEAD> of an Anno-J instance page, or including the URL to the CSS asset in the config file for the Anno-J instance.
If you do create a useful CSS please consider adding it to the list so that other people may use it.