If you are refering to timelines, you can use data-chrome=“transparent” which will remove borders and inherit your container background. Then you can style the iframe how ever you want. Here’s an example: http://codepen.io/indianburger/pen/JoXqPR
If you meant tweets, then it’s not possible at the moment.
EDIT: I thought you were talking about tweets, but I think you mean timelines. In timelines it is possible and I have edited my answer to show how.