Seems to work
(though I was afraid it was just a cosmetic trick).
Thanks Fizzlemizz.
Still another question:
I'm unable to check with LUA the following checked channels (sorry, my UI is french localized):
I tried
ChatFrame_AddMessageGroup(frame, value) with following values
"BG_SYSTEM_NEUTRAL", "BG_SYSTEM_ALLIANCE", "BG_SYSTEM_HORDE" and "BN_ALERT" without success.