How to customize twenty?

Is customization only in antimatter build in? How can I customize twenty the best way?

Just in case you haven't seen this little gem it might be helpful... http://getgrav.org/blog/theme-development-with-inheritance

Maybe you would like to add a custom stylesheet like you can do antimatter, is that correct? To do that, just inherit the theme, as @paulhibbitts suggested, then add your custom stylesheet in the stylesheets block, as follows:

{% block stylesheets %}
[...]
{% do assets.addCss('theme://css/cutom.css') %}
{% endblock %}

Yes! Thanks!