Follow System Dark Mode Preference with CSS
Use prefers-color-scheme media query and a manual toggle that persists to localStorage so the theme follows the OS unless overridden.
Task
Add @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } so the interface remains usable and accessible under the edge cases that manual testing skips.
When to use this
A screen reader user reports the page is unusable because @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } was never implemented, flagged in a support ticket.
Prerequisites
- A frontend component or page in the codebase
- A build with the relevant framework configured
Steps
1. Configure @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } in the project configuration file with the recommended defaults.
@media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } }
Expected output: The configuration is accepted and @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } runs without errors on the first invocation.
2. Run the check locally to verify @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } produces the expected result before pushing.
pnpm test
Expected output: Local run matches CI behavior; no surprises after push.
3. Add the gate to the CI pipeline so @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } blocks merge on failure.
- name: Run add dark mode system preference
run: pnpm test
Expected output: Pull requests with failing @media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } } are blocked from merging.
Troubleshooting
Problem
The check passes locally but fails in CI
Solution
Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.
Problem
Configuration is rejected with a schema error
Solution
Check the tool version matches the config schema; older versions may not support newer fields.
Limitations
- The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
- Configuration drift is possible if someone edits settings outside version control.
Handle the edge case
Start by measuring the problem. Use prefers-color-scheme media query and a manual toggle that persists to localStorage so the theme follows the OS unless overridden. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run @media (prefers-color-scheme: dark) { :root { –bg: var(–bg-dark); } } against the current codebase and inspect the output before wiring it into CI.
@media (prefers-color-scheme: dark) { :root { --bg: var(--bg-dark); } }Test with real tools
Once the gate is in place, make failures actionable. A failing @media (prefers-color-scheme: dark) { :root { –bg: var(–bg-dark); } } should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.