I believe there is an easier way to achieve this coloring without using multiple CSS files. You could simply make the selectors more specific so that each skin activates only one of them, for example:
You can add the other skins you are using with their custom colors. Note the nullifying of the heading at the beginning of the page. This is what enables you to remove the white space.
As for your later questions - I would ask you to explain in greater detail what you mean with "correctly" skinning the RadSpell dialog. This control has the design we have intended and I believe that it is correct. If you wish to modify if further you can use the DialogsCssFile property and provide a custom stylesheet.
If you have something concrete in mind I would ask you to send us a screenshot which displays the behavior you would expect as default.
Also what should the normal color of a textbox be? I believe white is the default one and it is so in the RadSpell. If you examine the rendered HTML you will see that the RadFormDecorator does not decorate the textboxes there. This means that you can provide your custom settings in the DialogsCssFile if you wish to modify our default ones to match your needs.
As for accessing the skin - for example in the Page_Load event of the page you can access the skin of the RadSpell by simply using RadSpell1.Skin.
the Telerik team