The current CSS editing implementation requires an editor like SublimeText or Notepad++ to edit the custom css, redeploy the application, and then check if the changes match. A WYSIWYG-type editor within Mendix would help streamline this process.
If you are using Chrome you can also use a plugin like CSS refresh. If you edit the web css file you can just right-click in your browser and choose Refresh CSS. When you are satisfied with the results you can copy-paste the css changes to the custom css file.
That's great to hear, Mike!
Jim, I think the forthcoming web modeler in Mx7 will do just this.