rainbow-dark: line_start uses ROYGBIV (7 colours)

R-Y-G-C-B-M was only six. Full spectrum now: red, orange (%y dark
yellow, renders brown-orange on most terminals), yellow, green,
blue, indigo (%b dark blue as the closest ANSI indigo), violet
(%M bright magenta). Stays ANSI-only so it renders reliably
regardless of 24-bit truecolor support.

.irssi/rainbow-dark.theme

@@ -62,8 +62,11 @@ abstracts = {
   ## generic
   ##
 
-  # text to insert at the beginning of each non-message line — rainbow dashes
+  # text to insert at the beginning of each non-message line — ROYGBIV dashes
-  line_start = "%R-%Y-%G-%C-%B-%M-%n ";
+  #   R = bright red, O = dark yellow/brown (closest ANSI orange),
+  #   Y = bright yellow, G = bright green, B = bright blue,
+  #   I = dark blue (closest ANSI indigo), V = bright magenta (violet).
+  line_start = "%R-%y-%Y-%G-%B-%b-%M-%n ";
 
   # timestamp styling, nothing by default
   timestamp = "$*";