I want to use the same theme(Porto Theme) for different websites (multi websites) with different styles on a single Magento installation.
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You can create a new theme that is a child of porto theme, assign it to your new website then change what you need in this child theme.
There you can find information on how to create a new theme with a specific parent theme : https://devdocs.magento.com/guides/v2.4/frontend-dev-guide/themes/theme-inherit.html