Hi,
In the directory where you have the webmail interface installed - WEBMAIL - there is a DEFAULT.ASPX file that generates the initial login page. Also, there is a subdirectory called AUTH, and in there some of the pages that make up the user-interactive portion of the web interface. You can edit those pages; make backups before modifying anything.
While there isn't a lot of documentation on editing the webmail interface, the files themselves are pretty well commented, and you should be able to change what you need. I haven't really looked at the skinning ability yet, apparently it is there. Hopefully we will start to see installable "skin" scripts soon.
As for the rich text option, there are drop down options below the compose window on the far right, which lets you specifty plain text/html or the encoding of a message. These options can also be set on a per-user basis via check boxes in the settings dialog as well, so for example you can default to HTML composition.
- Joe