My preferred desktop twitter client right now is the excellent DestroyTwitter. It’s got everything your need and is pretty nifty all round. In the latest beta you can customise the themes and as I was messing about with a few things I’ve created one for me that’s based on the scheme of this site. If you like those colours then you can grab the theme file. Just double click it and it should install. Here’s a little more about DestroyTwitter:

DestroyTwitter is a robust but compact Twitter application built to run on Mac, Windows, and Linux using Adobe AIR. It consists of a series of canvases that constantly update to keep tweets current and up-to-date using notifications that appear immediately after a new tweet arrives. DestroyTwitter also features complete direct messaging functionality. Messages and tweets can be replied to with the original visible for quick and easy reference. A search function is also available to track anything that's being talked about.</blockquote>