Define the preference order
A site can follow the operating system by default while allowing a person to choose a theme explicitly. Decide which choice wins before implementing the toggle. A practical order is an explicit saved preference, then the system preference, then the site’s default. Document that behavior so a later stylesheet does not quietly reverse it.
Make each theme complete
A light theme needs more than a pale page background. Inputs, borders, placeholder text, menus, code samples and focus indicators all need appropriate values. Use semantic custom properties for these responsibilities. For a product page, silver surfaces with plum text and gold accents can form a coherent light experience without inheriting dark header or footer panels.
Use the media query as a baseline
The prefers-color-scheme media feature lets CSS respond to a light or dark system preference. It can provide an initial theme without waiting for a component effect. If a manual choice is also supported, scope the system rule so that it does not override the explicit theme attribute after the person has selected a different mode.
Consider browser-provided controls
The color-scheme property tells the browser which schemes an element supports, affecting some native controls and other browser-rendered details. It does not recolor your custom components automatically. Match it to the actual page theme and inspect select menus, date fields and scrollbars rather than assuming all browser controls will follow custom variables.
Handle persistence gracefully
Saving a simple theme preference can make return visits more predictable, but storage access can fail. Catch that failure and let the switch still affect the current page. Avoid depending on user-agent detection or a large personalization script for a basic theme decision. Keep the saved value small and validate it before applying it.
Check first paint and navigation
Reload under each system preference and with each explicit setting. Watch for an unnecessary flash between themes, then navigate across routes and inspect the page chrome. Test screenshots and form fields separately. A successful theme system feels stable because the same preference is interpreted consistently before loading, after interaction and on the next page.
Further reading
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