Apple changed the looks of Notes.app in Mavericks and it is now mostly white with a light yellow paper texture as the note background. Turns out the texture is a TIFF file you can easily edit with any image editing app.

Quit Notes.app. Go to /Applications/Notes.app/Contents/Resources. Copy paper.tiff file to your desktop. Make another copy and save it as a backup to a safe place. Open Desktop/paper.tiff to Photoshop or some other image editing app. Use your creativity. After saving, drag the file back from desktop to /Applications/Notes.app/Contents/Resources.

I changed mine to a bit more saturated and removed the texture. Looks like a Post-It note.

Source: Mac OSX Hints