Using composer to process the new stylesheets and templates is a simple matter of getting to the root of your Chamilo code folder (where composer.json lies) and launching composer install. That is, if you have composer. If you don't, then you can also use the "Clean cache" option from the administration page's "System" block.
Clearing the template cache
Clearing the template cache should not be necessary if you have set your portal in « testing mode », but if you haven't, you'll have to clean the « app/cache/twig/ » folder.