Review the whole interface
Dark mode is not a simple negative version of a light page. Changing the background affects text, borders, cards, images, shadows, controls and every state between them.
A common problem is a dark main background with one section retaining a fixed light colour. Another is white text on a surface that also becomes light. These may look like small CSS mistakes, but they can make content difficult or impossible to read.
Contrast applies beyond body text
WCAG 2.2 defines minimum contrast requirements for text. Normal text needs a ratio of at least 4.5:1, while large text can use at least 3:1. W3C also explains contrast for important non-text elements, including boundaries and visual cues needed to understand and operate controls.
The ratios are a starting point, not the complete quality review. A control may pass one ratio while its hover or focus state remains difficult to see. Check every state: default, hover, focus, active and disabled.
A practical lesson from samthomas.no
During work on this website's dark theme, mobile screenshots showed sections that retained a light background while their text had become white. The main background had changed, but parts of the colour system had not followed.
That exposed a specific problem to fix: text and section surfaces needed to be considered together. It is why I check complete pages and component states when working with themes.
This is an experience from developing my own website. It is not a user study or evidence that the site meets every accessibility requirement.
A practical test
Review the complete site in both modes, component by component. Look for surfaces that still use fixed light or dark colours. Test text, links, icons, forms, menus, cards and images.
Then use the keyboard and mouse. Are focus and hover visible? Try a small mobile screen and enlarged text as well. Dark mode is complete when the whole task works, not when the background has changed.