Due to the nature of Magento, we’re unable to guarantee that all third-party extensions will work "out of the box" with Intenso. Unfortunately, it’s impossible for theme authors to make the theme compatible by default with all existing extensions.
The author of the extension is responsible for providing support in cases of compatibility issues. Each extension modifies Intenso/Magento functionality in a different way, so it's not up to us to anticipate how it will work in each possible case, considering there are thousands of available extensions for Magento.
When you plan to install an extension, the best option is to ask the author of that extension about the details since they will know better how the extension works, what elements of the theme overrides and how to integrate it with custom themes.
Only the "default" Magento theme is compatible with all third-party extensions because all extensions are designed to work with it. But authors of the extension should provide detailed instruction about how to integrate their product with custom themes since nobody really uses the "default" theme in production stores.
Generally, all extensions can work with all themes but they sometimes have to be integrated manually. That’s because sometimes the extension and the theme override the same template file, block or model which may cause a conflict. In such cases, you need to customize the theme and merge the two conflicting files into one file.
This isn't something specific to Intenso, this is a general rule for all Magento themes and extensions.