IMHO this is actually much more interesting than editing CSS files in Radio's outliner.
I see Radio as the "web embassy to your desktop". It can talk to the web (via XML-RPC, SOAP, HTTP, FTP, POP, SMTP), but it can also talk to your local applications, because unlike all other web apps, it runs on your desktop.
To edit our own content management system templates we have developed a Radio tool which download the template, converts macros to placeholders and allows you to edit html using any html editor. Once you are done all you have to do is save, the template will be compiled and sent back to the server, along with all its attachment (images, CSS, etc.).
But it can get much more interesting than that: you could edit any kind of content of your weblog using local specialized applications.
Creating a tool that allows you to edit on any image of your weblog simply by clicking it and having it opened with PhotoShop seconds later would be an easy task for Radio. Same thing for any piece of text (or, in perspective, audio or video).
So this could be my long term wish for UserLand: allow us to manage our weblogs on-line with Manila (doing it server side has some significant advantages), and use the full power of Radio to make integration with local applications a unique experience. I want to write with Word (or BBEdit, or anything different from the browser), edit my images with PhotoShop, organize my pictures with iPhoto, my appintments with iCal, but I don't want to spend my life uploading and downloading files.
PS: if this Radio/Manila integration could be done using some new or extended kind of open API which all developers could use, it could mean changing the world. Once more.