How to override Liferay core' css?

Hi there i'm want to override a data picker css from a hook creation. In Liferay Documentation 6.2 they say that it's possible with hook.

http://www.liferay.com/it/documentation/liferay-portal/6.2/development/-/ai/overriding-web-resources-with-hook-liferay-portal-6-2-dev-guide-en

But in Liferay IDE, when I create a new hook, in the interface for xml liferay-hook there isn't an option for CSS.

There are:

• Portal properties
• Custom JSP
• Service Wrappers
• Filter and Mappings
• Struts Action
• Indexer Post Processors
• Language properties

So, how to make it? Thanks ahead!

you need to create a plugin: THEME. You can see how in this page: https://www.liferay.com/es/documentation/liferay-portal/6.2/development/-/ai/creating-themes-and-layout-templates-liferay-portal-6-2-dev-guide-09-en In this plugin you have a folder _diffs. Everything you put here is going to overwrite liferay´s theme.