There are times that a theme is prompting for an upgrade to an accessible theme, but even after the upgrade, the theme still prompts that it needs to be updated. This rarely happens and usually occur to very old themes. Upgrading a very old theme will require few additional steps.
Upgrade to an accessible theme
Go the Theme Editor
Select the Theme that you're having troubles upgrading, click the Edit button
Scroll down the sidebar to the bottom and expand Legacy Variables
Set Text backwards compatibility to False
Upgrade the theme and click Publish