Windows – Changing Windows 8 file type icon

I'm having trouble changing the icon for specific file types in Windows 8. I have read countless how tos including this question and this one, tried FileTypesMan and several other programs and edited the registry but none work exactly. After each method I have purged the icon cache and restarted just to be sure. I want to change the icons for .css and .js files (and others down the line) to separate icons, but both types open in Notepad++ by default.

Changing the DefaultIcon key in the registry for HKEY_CLASSES_ROOT\Applications\notepad++.exe (the ProgId associated with .js and .css in HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.css\UserChoice) changes the icon for all files that open in Notepad++.

Changing the DefaultIcon for .css or .js files in HKEY_CLASSES_ROOT\CSSfile and HKEY_CLASSES_ROOT\JSFile has no effect.

Adding a DefaultIcon key to HKEY_CLASSES_ROOT\CSSfile did nothing either, and I was reluctant about experimenting by adding it elsewhere in case I broke something.

One thing I have noticed is, if I change the icon of an extension without a default program associated with it (.php in this case) then changing DefaultIcon in HKEY_CLASSES_ROOT\php_auto_file did work, but I stupidly chose to always open PHP files in Notepad++ after that and then it changed to the Notepad++ icon.

So is there a way of changing the icon for a file type that has a default program associated with it without changing all icons for files that open in the same program? Or is there a way of disassociating a file type with a program so that I can change the icon but always have to use "Open With…"? Preferably the former option.