Development overview
Mibew Messenger is a web application which uses PHP, MySQL, and JavaScript. If you want to patch the Core of Mibew Messenger and/or change the appearance by creating your custom theme you will also need node.js and npm to build the sources.
All source code of Mibew Messenger is available on GitHub: https://github.com/Mibew/mibew. All official plugins, themes, and translations for Mibew Messenger are available on GitHub too.
Take a look at Mibew organization on GitHub to get the list of all official repositories.