The colours probably aren't hardcoded into the code. With a lot of generator layouts, you'll put your colours in under the 'look and feel' tab of the 'customize journal' page. You'll select 'custom colours' and pick the ones you like. This image from LJ can give you a good idea of what controls what. For a monochromatic colour scheme like that, just pick a base colour and pick your other shades as either lighter or darker versions of the original.