The code has only been tested with Fluid, but should work in Safari and other WebKit-based browsers (including the iPhone I guess).
Put all Twoot files in a folder somewhere. Download Fluid and create the Twoot.app by following the instructions here:
Launching the app will prompt you for your username and password on the first run.
The default icon was found at http://www.iconarchive.com/show/dragon-soft-icons-by-artua/User-icon.html and is used according to the restrictions described (free for non-commercial use).
The styles/default/Morpho-Helena-128x128.png icon is created by Adrian Melsha and is used in accordance with the license terms here: http://www.iconarchive.com/category/animals/morpho-butterfly-icons-by-adrian-melsha.html
The idea is that you should adapt Twoot to fit your requirements. If you know HTML and CSS it is trivial to add a new style. Create a new folder under the style directory and add all style files (css/images) there. Don't forget to change the stylesheet link in the twoot.htm file.
Currently Twoot has two themes, "Default" and "HUD-black" to match the window styles available in Fluid. To switch theme, edit twoot.htm and point the stylesheet to the correct css file in the styles folder.
Bugs, praise, feature requests
Create a comment on the initial announcement page.