thunderbird - reply window

[Norman Silverstone]

I have Thunderbird on 2 machines. On one of them the message reply 
window has the text in blue and the lines of that message are marked 
with a > or >> and so on. On the other machine the text is black and 
lines of the message being replied to are marked by a coloured vertical 
line. The first machine mentioned used to be like the second one until 
something happened to change it.

Is there anything I can do to recover the reply window message marker to 
the vertical line and the text in black, please?

Norman