Snippet_Plugin
Problem: you need to change any (almost any) public method.
Here you can find more about plugins.
To enable this module execute:
php bin/magento --clear-static-content module:enable Snippet_Pluginphp bin/magento setup:upgrade
Snippet/Plugin/Plugin/Magento/Theme/Block/Html/Header.php file is an example of around and after plugins. 'Around' does nothing. While 'after' wraps into parentheses welcome message like this:
(Default welcome msg!)
Snippet/Plugin/Plugin/Magento/Theme/Block/Html/Breadcrumbs.php file is an example of 'before' option. E.g. on detail product page breadcrumbs will look like this:
(Home) > (Test Product)