process.env.REACT_APP_THEME_API_URL
..env
file and inside this file include a variable with name REACT_APP_THEME_API_URL and set value of your API URL.REACT_APP_THEME_API_URL=https://preview.keenthemes.com/theme-api/api
.env
files usually are hidden by default, to view them you can open a project in your code editor program or configure your finder to show hidden files or view them in terminal with command ls -a
.