Themes are basically just web pages. To create your own look and feel is as easy as creating any web page. Most of the work goes into creating the graphics, or deciding what the java client should look like. To begin I would take a look at the param file that is in the Application note section. I would also take a look at the variables section as that will allow you to create feature rich pages.
Another decision is if you want to have a param sheet or put everything into HTML params. A param sheet is a server side parameter file (ssp). It's handy for editing multiple chat pages at one time. Say that you wanted to add a send button to 10 chat pages. If those chat pages were coded to look at the param file then you could add it one time in the param file and it would change all the pages that reference it.
Now you can either place your chat page as the default.htm page in the data or htdocs directory which will then override all of the theme logic, or you can create severl different 'themes' and place them in new directories under the data/themes directory. Then you can use the /?theme=<directory> to reference that theme. To do this create a new directory and place a default.htm file there. Then reference it with the theme logic above. In this way you can have multiple themes for a server.
The easiest way to create a theme is to copy the default theme. It's already feature rich and should be pretty easy to edit to do what you want. However, if you find another theme that you like then use that as the base.
Create a new directory with the name of the theme that you want.
Copy all the files in the theme that you have chosen as closest to the one that you wish to create.
Look inside the default.htm file and find the param called ssp. Then go to the /data/params directory and copy the file that's listed in that param to a new name. As an example the default theme has an ssp file called def.prm. If your new theme is called MyTheme then change the def.prm to mytheme.prm.
Go into your new theme and open the default.prm then change the name of def.prm to mytheme.prm
Now you can start to change your new theme by editing the default.htm file and the ssp file.