In vaadin is possible change a portlet skin in the portlet code. But if I’d want change portlet skin directly from a portlet container like liferay, how can I do this?
For example in liferay portlet management I want to change vaadin text label font.
The portal theme settings will also affect the components inside a Vaadin portlet as long as the portlet is not running in an IFRAME. Therefore, you should be able to use Liferay themes to extend or modify Vaadin component appearance, using “.v-thecomponent-*” CSS selectors, and making the CSS rules specific enough that they override the default rules. You might need to e.g. start with “.v-app .v-thecomponent-…” to make your rules more specific.
If you want to switch to another Vaadin theme instead, you would need to somehow get hold of the Vaadin Application instance in Liferay.