In the HTML Editor Manager - you can setup the layout of the Toolbar by using the toolbar configuration tab - this tab displays the XML layout of the toolbar as use by the radeditor.
There is NO editor.css file by default - its a css file you create and put in the skin folder - so that its is skin specific - its up to the skin developer or yourself to create these as each site / skin can be entirely different.
Once you create this editor.css file with the styles you want to user on your site - you tell the radeditor to use it by setting the value of
CSS File: in the HTML Editor Manager to "/editor.css" - note the "/" at the front is important - this tells the radeditor to user the editor.css file that is located in your SKIN folder as the layout stylesheet for the editor window.
If you dont do this - the radeditor will attempt to make a guess at what styles should be used in the editor window - but it is not always a good guess - since it is based usually on default.css as opposed to your skin.css. This is the point of creating an editor.css file - it lets you override this default guessing and define exactly what styles you want to use.
The thing to understand is that the radeditor works in an IFRAME - and effectively only uses ONE stylesheet to control how it displays - in most sites the actual layout is a composite of Default.css, Portal.css, Skin.css, containner.css and even maybe module.css.
What you want to do is create an editor,css file that brings all the elements of STYLE together into one layout file.
As for where the provider is stored - thats a factor of how the HTML Editor Manager works - it is a module so has been placed in the /DesktopModules path.
Westa