Pick a Color from the frontend and config a json for the theme to be rendered. Then the css file on the server will be modified with the new values. Using Nuxt and express.
# install dependencies
$ yarn install
# serve with hot reload at localhost:3000
$ yarn dev
# build for production and launch server
$ yarn build
$ yarn start
# generate static project
$ yarn generate
For detailed explanation on how things work, check out Nuxt.js docs.