Start with the default themes Olivero and Claro, both of which are built to WCAG 2.1 AA standards. If building a custom theme, use semantic HTML, proper heading hierarchy, and sufficient colour contrast.
Install the CKEditor Accessibility Checker to help editors create accessible content. Use the Accessibility Toolkit contrib module to surface issues in the admin interface.
For images, require alt text through field configuration. For forms, make sure labels are properly associated with inputs. Test with keyboard navigation, screen readers like NVDA or VoiceOver, and automated tools like axe DevTools. Accessibility is not a feature you add at the end but a process throughout development. Drupal gives you a strong foundation, but achieving real accessibility requires ongoing attention from designers, developers, and editors. The effort pays off in reaching more users and often improves usability for everyone.