I am not sure if this is a bug or a feature request. I have marked it as a bug only because the online help is also not clear on it, so if it can't be changed in software, it should be better documented.
In the settings for HTTP Server (custom) it looks like the dialog box would allow changing the root directory. However, it does not allow typing in the field.
I would like to have different root directories for different projects, and also want to make sure that any edits I do to the default.html are not over-written by ARC updates.
My workaround would be to make default.html just be a simple menu to select a project and then navigate to a page in the project's folder, but I would like to avoid that step if possible.