Publishers can always enter into the Facebook UI to apply more specific styles. This is considered best practice when setting up Instant Articles for the first time and should also be audited with some frequency when new features are released to optimize the visual presentation in this channel.
The below are the high level steps to navigate the Facebook UI and make these types of design changes/optimizations:
Login to Facebook > Manage Page > Publishing Tools > Facebook Instant Articles > Configuration > Styles
The Styles group will have a default template applied. Select it. This will pull up the full Style sheet for that publisher's Instant Articles.
Scroll down to Quotes area where you can see the defaults for Blockqutoes and Pullqutoes, a preview in the right hand side, and can edit these defaults. When done making changes click on "Save".