With the last commit in master in late 2018, SemanticUI seems a bit deserted.
Anyway, in order to change it, you should discard everything non-nextjs and non-docker related from the client/ folder, like client/common/, client/components/, client/helpers (except client/helpers/configureGraphQL.ts), client/icons, client/layouts, client/theme, client/pages/{login.tsx|signup.tsx|dashboard/}, remove the material-ui related stuff from package.json and then proceed installing the SemanticUI.
