MacMillan developed user customization enhancements for the zen-browser/theme-store repository, focusing on improving theme management and user control within the browser interface. The work centered on a settings feature that allowed users to specify the scope of the close button removal, distinguishing between all tabs and pinned tabs, and introduced a theme customization enhancement with a preferences link and version update. Using CSS for styling and JSON for configuration, MacMillan implemented semantic versioning and incremental releases to ensure maintainability and traceability. The changes consolidated customization flows, improved accessibility to theme preferences, and enhanced the overall user experience without addressing explicit bug fixes.