You can provide inline styling for any component by using the style property.

You can also load your own stylesheets, either by providing your custom html or by adding stylesheets from your java code, either declaratively or imperatively.

All the design systems used in Mateu provide also their own css variables, the default values of which you can easily override using css as explained at https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascading_variables/Using_CSS_custom_properties

E.g., for Vaadin design system you can find the list of available css variables at https://vaadin.com/docs/v25/styling/lumo/lumo-style-properties while for the Red Hat design system you can find the list of available css variables at https://ux.redhat.com/tokens/.