Shopware 5 Theming 101

First things first:

1. In the Theme Manager in the backend add a new theme that inherits from Responsive
2. It will be saved to themes/Frontend/Mytheme
3. To add your own stylesheets to your theme put a file called all.less here: themes/Frontend/Mytheme/frontend/_public/src/less/all.less. It will automatically be compiled by Shopware’s own LESS processor
4. To add your own scripts to your theme put them in themes/Frontend/Mytheme/frontend/_public/src/js/ and them to the $javascript array in themes/Frontend/Mytheme/Theme.php (snippet below)
5. To immediately see your custom scripts and styles disable compiler cashing and CSS and JS compression in the settings of the Theme Manager

Adding a JavaScript file to your Shopware 5 Theme:

<?php
/**
 * Javascript files which will be used in the theme
 *
 * @var array
 */
protected $javascript = array(

    // Third party plugins / libraries
    'src/js/app.js',
);