Worked on the mermaid-js/mermaid repository to deliver theme-aware rectangle backgrounds for sequence diagrams, ensuring that default colors are applied when users do not specify their own. This approach improved visual consistency and reliability across both base and dark themes, reducing user confusion and enhancing the overall experience. The implementation leveraged TypeScript and JavaScript, with a focus on front end development and thorough unit testing to validate rendering across themes. Incorporated feedback from code reviews to refine the solution, resulting in maintainable and robust cross-theme support. The work emphasized careful testing and documentation to support ongoing development and usability improvements.