I want to change background colour on reply post, see pictures.
can you elaborate - do you want main post in one color AND all the replies in a different color ?
Yes, I want to change just background colour to all reply post.
You need a plugin for that. Or you can opt to install
PHP in Templates and use a conditional:
<if $postcounter == 1 then>
// code for the first post
<else />
// code for the rest of the posts
</if>