You can add some background selections by using Page-TypoScript/tsconfig.

TCEFORM.tx_masonry_domain_model_masonryitem {
  background_style {
    addItems {
      gray-light = Gray light
      gray-dark = Gray dark
    }
  }
}

Removing background style selections:

TCEFORM.tx_masonry_domain_model_masonryitem {
  background_style {
    addItems {
      white >
      black >
    }
  }
}

Finally you will get a CSS class on masonry item, which is named by your selection key. For the gray-light key the CSS class will be masonry-item-background-style-gray-light.