Check out my collegue Stefan's blog post: http://world.episerver.com/Blogs/Stefan-Torstensson/Dates/2012/12/Adding-a-new-device-to-the-view-resolution-drop-down/

Thanks! It took some time to figure out that the path is relative to ClientResources but now it works like a charm!

I fail to see the connection between adding new device in view dropdown with replacing default tree icons with custom ones? Can you please clarify a bit?

It was the part about adding the displayresolutions.css to clientResources that I could use for my own style sheet, ie this code:

<clientResources>
    <add name="epi.cms.widgets.base" path="Styles/displayresolutions.css"
resourceType="Style" isMinified="false" />
</clientResources>

The blog post about display resolutions explains how you can load a custom css file to the user interface. This can be used to add custom classes/icons for both display resolutions and content icons.

Thanx for the reply. I got that part so far. There is one thing that puzzles me though. The module.config and ClientResources exist on at least 3 locations within Modules, ModuleRepository, somewhere in Shell.

Where exactly to add custom .css file and which exactly module.config to use to register new .css?

You should add the css declaration to one of your own module.config files. If you look at the Alloy sample packages (both Web Forms and MVC) there is a client module in both projects with a module.config file placed in the root directory. Both JavaScript and Style sheets should be placed in the ClientResources folder for this module (check the registration for Styles/Styles.css).