How to use Stationery in a Reply

Unfortunately there's no way, in Thunderbird, to apply a Template to a message you're already replying to.  You can only use a Template for a new message.  But there are two ways you can dress up outgoing replies.  

1.  Go to Format - Page Colors and Background and choose a light-colored texture graphic file.  Thunderbird remembers the last directory you browsed to, so if you make a small collection of suitable background graphic files in a directory just for this purpose, it's not too inconvenient to do this all the time.

2.  To use the sender's stationery, instead of clicking Reply, click Edit As New.  That preserves the background image.  Then Select All, Cut, Paste As Quotation.  That makes the sender's words indented like a normal Reply and you can go from there.  You'll have to put the sender's email into the To: box and add a Re: to the subject line.  

For the first option, here are some good light background tiles I've gathered from incoming emails over the years!