How to change background for a specific position in T3 ?

Hi everyone,
I have a problem of finding a way to change the background of an specific position.
I want to change the background of the outline of container.
I don t want to change the container background. Only what s out of the container.

Please view the picture for more details.
Sorry for my bad english.
There are 3 pictures.
1) what i want
2) what i would like to do
3) what i don t want.

Any idea ? How can i do it ?