Default.skin refresh

On a side note, I originally had the "ProjectName" folder in angled brackets, not square brackets. The full question appeared fine in the preview, but wasn't visible after saving.

Typically when you setup your site and use the CSS Stylesheets app, a folder is created in the /App_Themes/< csscodename > directory. In there you create your default.skin file and style the calendar as you see fit. Best practice is to name your stylesheet code name the same as your site name so it all exports if and when you choose to export your site.

You can then set a SkinID in the .skin file or leave it empty and it will be automatically applied to all calendar objects. I'd suggest adding the ID then in your webpart configuration, you enter the same SkinID and things should link up.

The folder was there. The skin file was missing. I created it using a copy from one of the stock templates to make sure I structured the file correctly and spelt the file name exactly right, but still, the Default.skin file doesn't seem to be looked at.

Does a link to this new file need to be defined somewhere in the Site Manager? A config file on the server?
Does the application need to be rebuilt in Visual Studio before changes can be seen?

Look in your newly created .skin file (should be named "Default.skin") and look for a SkinID property, if it has one, then you need to apply that SkinID to your webpart as well. If you want it to be applied to all controls no matter what, remove the SkinID property.

Also it could be that you're trying to reference a standard asp.net control <asp:Button> and you need to reference the Kentico version <cms:LocalizedButton> in your skin file. Make sure the control is the same. You should not have to recompile or do anything else, it should pick up automatically.