You are using still WinXP? Just tested in my WinXP VM and it works, too! This is how it looks on my Win8.1: The bitmap file is in your script dir? Br, UEZ
Guess you are missing what Style means here
The Lexer assigns one of the below known AU3Styles to each character in the script. Only the matching style characters will be seen as Delimiter.
So 0 is any comma and 8 in this case is any comma that has the Operator style.
# White space
style.au3.0=fore:#000000
# Comment line
style.au3.1=fore:#008000,italics
# Comment block
style.au3.2=fore:#008000,italics
# Number
style.au3.3=fore:#0000FF
# Function
style.au3.4=fore:#000090
# Keyword
style.au3.5=fore:#0000FF
# Macro
style.au3.6=fore:#808000
# String
style.au3.7=fore:#FF0000
# Operator
style.au3.8=fore:#FF8000
# Variable
style.au3.9=fore:#5A5A5A
# Send keys in string
style.au3.10=fore:#808080
# Pre-Processor
style.au3.11=fore:#808000
# Special
style.au3.12=fore:#DC143C
#Abbrev-Expand
style.au3.13=fore:#FF0000
# COM Objects
style.au3.14=fore:#993399
#Standard UDF's
style.au3.15=fore:#0080FF
All clear?
Jos
If after all of that you still want more control then download SciTE Customization GUI found in my signature. This utility will give you total control over the theme and many other relevant SciTE properties and SciTE4AutoIt3 settings too.
See my signature for a toolbar add-on for SciTE, there are also others that would work for it too. Not natively, there might be an add-on that will do that, or a rewritten version that has that. See above.