How can user-defined portlet styles be incorporated into Liferay themes?

User-defined portlet styles can be effectively incorporated into Liferay themes primarily through the application of CSS styles in portlet decorators. Portlet decorators allow developers to customize the look and feel of individual portlets or collections of portlets without modifying their underlying functionality. By defining specific CSS classes or styles in the portlet decorator, designers can tailor the visual presentation to match the overall theme or meet specific branding requirements.

Using CSS in this manner offers flexibility and aligns with the principles of separation of structure and presentation, enabling developers to manage design elements efficiently. This approach leverages the existing mechanisms within Liferay to ensure that the styles can be applied uniformly across various instances of portlets, enhancing consistency in the user interface.

Other methods mentioned, such as configuring default portlet settings or utilizing JavaScript libraries exclusively, do not provide a direct means to style portlets in the context of themes. Disabling theme settings would generally remove styling rather than contribute to it, making these options less relevant for incorporating user-defined styles into Liferay themes.