On separate layers under the "LineArt" layer, a border and sky are added. The border was created with Photoshop 5.5 and the sky was scanned from an old black and white engraving in a Dover book (Dover books have copyright free, old time engravings).

The black lines of the sky were colored by deleting the white pixels, leaving only the black pixels of the line work. Preserve transparency was then checked for its layer and the lines filled with a gradient, using the Gradient tool set to "Foreground to Background."