@bkart,

you can use the .img-metadata label selector to modify the background and border. We recommend to use the child them stylesheet to overwrite the properties.

To display the image description under each image you can uncheck the setting in settings > gallery > “Use title as caption”. At the moment you can only use one or the other but you could add the title to the description to display both.

@Engholm,

You can added using your child theme or a custom plugin just use the wp_enqueue_style() function. If not you can always add additional rules to the style.css in your child theme.