You want something dimmer than the indigo (#6366f1) I showed. Here are darker, more muted options — each shown on a task list mockup.
Muted blue-gray. Accent: #4b5ea8. Very subdued, professional. Close to VS Code's dark theme feel.
Muted purple. Accent: #7c6aad. Slightly warmer, still understated. Has a subtle "Claude" vibe.
Muted teal-green. Accent: #4a8c8c. Cool and calm. Distinct from typical blue-heavy dark UIs.
Desaturated steel blue. Accent: #5571a1. Very close to Microsoft To Do's dark mode accent but dimmer.