Custom CSS on the widget

We expose three customization layers: theme color, position, and Custom CSS (Business plan). The Custom CSS is sandboxed to the widget shadow DOM, so you cannot accidentally break your site styles. Examples and snippets at /help-center/chat-widget/widget-customization.

We override the launcher position to top-right on mobile only. CSS media queries work inside the widget custom-css block?

Yes. Standard CSS. Use @media (max-width: 640px) { ... } and it will only apply on mobile.