XF 1.5 Add a border beside the avatar block

[Betclever]

Hi,

I would like to add a border beside the avatar block so just between the avatar block and the message block.
I tried on my side but the line doesn't go to the end of the block and there is a kind of white space...

See capture:

https://gyazo.com/36cb2f77ff54c824b53ee52f5dacfe35

Do I have to create a new div?
If yes, where please?

TYVM.