To create and manage themes, you can navigate to Customizations -> Themes in the FusionAuth UI or API. More here: https://fusionauth.io/docs/customize/look-and-feel/

Themes are written in FreeMarker, which provides a rich template language to customize pages and helpers to suit your needs. This could include adding accessibility features to meet WCAG standards.
If you need to manage complex configurations such as themes programmatically, you can use the FusionAuth CLI.

If you're looking to ensure that your custom theme meets WCAG requirements, you might need to consult WCAG guidelines and ensure that your theme modifications align with those standards. This could involve adding appropriate ARIA labels, ensuring sufficient color contrast, providing keyboard navigation, and more.