How to not make my forum use multi-toned threads

On my forum every other post has a different background

So the background in post #1 in the thread is one color
Then the background in post #2 in that thread is a slightly darker version
Then the background in post #3 in that thread is the same color as post1
Then the background in post #4 in that thread is the same as post2


How can I make it so that every post uses the same background:
Here is an example image:



The image I posted was just of my badges and the forum line separator. In post #1 it all blends in and you can only see the line and badges. In post #2 you can see the box of the image because the background is not the same.

All help and replies are appreciated