Question: In previous versions of IE developer tools you were able to open the source of CSS files in the debugger. 

This was of most use to verify that they had actually been downloaded / referenced correctly, ie if the request for a stylesheet returned a 404 error then thats the route cause of my styles not being applied.

However as of IE 11 the debugger no longer lists CSS files, so I can't use that to view their source and verify them.

I know that I can do this through the profiler - but that requires starting a profile trace and reloading the page, which is not exactly conducive to easy work flow. (Not to the frequent lock ups the F12 tools seem to experience on any of the profile tabs is used)

Is there still away through the dev tools to see the source of CSS files?

Answer: 
The CSS tab is missing, and the debugger does not list CSS files either.

However, If you find an element in the dom explorer with styles applied from a css file, you can then click on the file name and open the entire file.
In developer tools, go to the Network tab. Click the arrow (when you hover over it, it will say "Enable network traffic capturing"). It will turn into a square to indicate that it is capturing network traffic. Refresh the page. Click type to sort by type and look for "text/css". Double click the file. Click "Response body". You will see the CSS contents there.

To manually add styles, go to the DOM Explorer. Click an element in your webpage and it should bring you to that part of the webpage. You can add styles to the elements in the DOM Explorer or in the left window, you will see "Inline style" (if there are no CSS classes) or you'll see "Inline style" as well as the CSS class and its details. You can click in there to make changes.