if you need to add CSS to change the layouts and you cannot achieve that by changing to a new gallery style, there are more gallery styles and options with either the EXPERIENCE add-on Or a Premier subscription.
Layout Documentation under Experience.
The developer customizes new styles for Premier and as part of his paid customization projects on a regular basis, but since there are so many themes out there, he cannot customize one size fits all for every one of them. There are quite a few different gallery styles available to you, but as mentioned the ones that work best with responsive design may require an add-on.