This page consists of screenshots from the Layout Settings – Custom CSS Screen in Gallery Master WordPress Plugin.
This gives an idea about how to add CSS.
Custom CSS allows to provide CSS code manually to add extra styling to the layout of Gallery Master. CSS style will override the plugins css and provide you the ability to customize the settings for various layouts in Gallery Master.
Once its done, then you would need to click on “Save Changes” button located at the bottom & top of the page to save the changes.