There are currently issues with MUI and Next.js around theming. The MUI theme functions occur on the client, which makes it impossible to use them with server-side rendering. This open Material UI PR outlines the issue.
Until the issue is resolved, Next.js files in the app directory require
"use client" to be at the top of any file that references the theme. When the
issue is resolved on the MUI side, that line can be removed.