You can currently change your logo (account wide), the cover image (per gallery), the text above the gallery (per gallery) as well as the size and type of thumbnails and of course the functions of a gallery. You can also format the description text of a gallery and include Youtube or Vimeo videos.
The features of your gallery also include (from the Pro plan upwards) the Presentation Mode, in which you can choose from a lot of design settings for your galleries (per gallery). For example, you can adjust the background color or the font according to the style of the gallery. Please find detailed information here.
Use the welcome text to show your clients how to set image flags and create image selections or to link to your custom instructions.