Included in the download are HTML demo files that can be a pretty good way to try out Galleria without coding from scratch.
Galleria has a large number of options such as transition effects, cropping methods and interaction details that you can use to customize your gallery regardless of what theme you are using. Read through the options documentation for full coverage. In addition to the generic options, each theme has some theme-specific options. Read more in each theme documentation available in the docs.