Identifying themes that need to be tested
There are two ways to find themes that need an accessibility ready review. How you find them varies based on if the theme has already been published in the Themes directory or if it is still under review.
New Themes
Themes that are not yet approved by the themes team and that have added the accessibility-ready tag can be found in the Themes Trac.
If you choose to review one of these themes, leave a comment on the thread noting that you are reviewing the theme for accessibility so the assigned reviewer from the Themes Team, the developer, and other people from the Accessibility Team are aware that it is getting reviewed.
Existing Themes
Developers of themes that are already live request a review via a form on the accessibility team website (log in required). Form entries and their status are tracked in a Google spreadsheet: WordPress Theme Accessibility Ready Theme Review Requests.
If you choose to review one of these themes, change the Status column for the theme to Reviewing and add your WordPress.org username in the Assigned To Username column.
WP Accessibility Knowledge Base