examples of CSS rules that will take precedence over the built-in SLP Theme file CSS and WordPress built-in CSS. can be found on the docs page. To find the correct rules, the developer recommends using the Firefox Developer tools or Google Chrome Developer Tools to view and copy specific CSS paths.
The Docs page shows you how to change the span and css.
Also, there are a few other SLP plugin styles, left for red, left for blue, and other Gallery styles available for premier customers. You might want to check them out on your dev site, make sure you have a back up of your site before changing anything permanent.