We implemented a dark theme for our Singlestore Helios Portal UI, allowing users to select between light, dark, or system (operating system) themes. The implementation involved providing logic and UI for getting and setting the applied theme in-app and in Storybook, extending design tokens and migrating to CSS Custom Properties, conditionally switching between light and dark graphics, testing and quality assurance, revising our design tokens and component library, discovering edge-cases and collaborating with designers to decide on a solution. We also implemented typed styling props using Class Variance Authority, accessing token values in TypeScript files, generating code derived from design tokens using Style Dictionary, and providing utility classes for applying common styles. The final result was shipped to customers after extensive testing, demonstrating the benefits of implementing a dark theme with CSS Custom Properties.