Custom Newsletter template
You can include standard HTML in Twig templates. So something like this would work:

&lt;table style="background-color: gray"&gt;


You don't have to use Twig filters to add CSS or CSS files. Although you could create some CSS variables using {% set gray = '#eee' %} or {% set border = 'border: 1px solid black' %} for repeated styles such as colors, borders, etc., and then reference those like this, using the variable name:

&lt;table style="background-color: {{ gray }}; {{ border }}&gt;