Although, you may not comprehend it, it is very important to match the curtains in with the theme. This does not mean that you have to play it safe and make the curtains match perfectly but they should not clash with the custom furnishings. 
Make sure that the materials reflect the custom furnishing materials in the room, the color works with your color palette and the thickness of the material works with the feel of the interior decoration. The last thing you will want is a heavy drape in a light and airy in your home design space.
When it comes to the color, you can use the curtains to add depth to a space or you can use the curtains to provide a little pop of color or a statement piece to the room.
Last of all, I would suggest sketching out the window area and drawing in the drapes before you go out and purchase them.
