The default Lucy contains a menu bar, several tool bars and pop-up menus. It is possible to customize those menu and tool bars. For example, you can remove entries, change an icon, provide another tool tip, and so on. To solve the most common and basic use cases, you can adjust configuration files. No coding is required.
Other modifications require knowledge and understanding of the Lucy action bar API. For example, you need the API to add your own entry to an existing tool bar, or to create your own tool bar and populate it with your own entries and entries already present in Lucy.
Both scenarios are documented here.