Syntax

Here is a formal definition of the syntax that I came up with using EBNF. I didn't bother with a proper tutorial because you could already infer everything from the provided default input.

six-letters = "A" | "B" | "C" | "D" | "E" | "F"
            | "a" | "b" | "c" | "d" | "e" | "f" ;
hex-char = six-letters | "0" | "1" | "2" | "3"
         | "4" | "5" | "6" ;

hex-three  = 3 * hex-char ;
hex-six    = 6 * hex-char ;
hex-colour = "#", hex-three | hex-six ;

newline = "\n" ;
letter = six-letters 
       | "H" | "I" | "J" | "K" | "L" | "M" | "N"
       | "O" | "P" | "Q" | "R" | "S" | "T" | "U"
       | "V" | "W" | "X" | "Y" | "Z" | "g" | "h"
       | "i" | "j" | "k" | "l" | "m" | "n" | "o"
       | "p" | "q" | "r" | "s" | "t" | "u" | "v"
       | "w" | "x" | "y" | "z" ;
word = letter, { letter }, newline ;

colour-property = ( "fg" | "bg" ), ":", hex-colour ;
opt-property    = "opt:", word ;
property        = "+ ", ( colour-property | opt-property ), newline ;

element = word, { property } ;
input = element, { element } ;