Every theme is kept in its own directory. The directory must contain a file called "layers.txt".
Reserved words are:
$luminary - when it is day, the variable has value "sun", at night it contain the moon phase ("newmoon","waxcresmoon", "fqmoon", "waxgibmoon", "fullmoon", "wangibmoon", "lqmoon", "wancresmoon")
$sky - contain the type of clouds
$season - contain the season
$daynight - contain "day" during day and "night" during night
$weather1 - first weather layer
$extra - layer for special occasions like Christmas or Halloween 
$weather2 - second weather layer
$nightshade - layer with semi-transparent black rectange, which makes the layers under appear darker. It is added only when it is night. 
In the layers.txt file the layers are kept, separated by "|". Every layer can contain one of the reserved words/variables, constant string or combination of them. 
