Dark mode is not a negative copy of a light page.

Changing the background also affects text, borders, cards, images, shadows, buttons and interaction states. A good result requires assessing the colour system as a whole.

Small inconsistencies can cause substantial readability problems

A common error is a dark main background with some sections still light. If text has also changed to white, parts of the content can become very difficult to read.

This may look like a minor CSS issue, but users experience a direct accessibility problem.

Contrast applies beyond body text

WCAG 2.2 describes minimum text contrast. The general rule is at least 4.5:1 for ordinary text and 3:1 for large text.

W3C also sets contrast requirements for important non-text elements, including visual cues needed to understand and operate controls.

The numbers are a minimum, rather than a complete quality check. A button may meet contrast requirements normally while having a weak focus indicator or unclear disabled state.

The theme needs to work in every component

While developing dark mode on samthomas.no, a mobile review showed some sections still had light backgrounds after their text had turned white.

The error made one thing clear: changing the main background is insufficient. Text, section surfaces and component states need checking together.

This is experience from developing my own website, not a user study or evidence of full WCAG conformance.

Images need assessment too

An image or illustration working well on a light background may have entirely different contrast or visual weight in dark mode.

Test transparent logos, screenshots, charts and decorative elements in both themes. Some may need separate versions.

Test the entire task in both themes

The check should cover more than the homepage.

Review text, links, icons, forms, menus, cards and images. Use a keyboard and mouse, checking clear focus and states. Test on a small screen and with enlarged text too.

Dark mode is finished when the interface works as a coherent solution, rather than when the background has become darker.