Refer below link(Point no-9) :- https://docs.mendix.com/howto/front-end/customize-styling-new
By default, Mendix generates the index.html (the page that is loaded to start the app), based on the app configuration. In some cases it may be needed to customize this HTML, which can be done by creating a file called index.html in the theme/web folder. To make sure that your file has the right structure, we recommend you copy index-example.html from the deployment/web folder to the theme/web, rename it to index.html, and then use it as a starting point. This file will be created after you have deployed your app locally at least once.