how to override the timeline-twitter css
In order to be able to style it, you'll need to stop using the standard twitter embed code and use another solution.

I suggest using the Twitter (https://www.drupal.org/project/twitter) and Oauth (https://www.drupal.org/project/oauth) modules for this.

This will allow you to create a view of your feed which you will be able to style easily.

There looks to be a pretty good recipe here: http://polso.info/integrating-twitter-feed-your-drupal-site

Good luck!
Anne