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If you want to add any custom css code to the extension, please go to Backend > Joom Donation > Configuration and open tab: Custom CSS and enter your custom CSS.
Joom Donation is a standard Joomla MVC extension so it works based on standard Joomla rules. Joomla allows you to overwrite output of any components and modules from the template (Checkout more about this here: http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core) so Joom Donation also allows you to do that easily.
Example: To overwrite output of product details page, you need to copy file: components/com_jdonation/view to folder templates/your_template/view and modify this file from the template.
Once the file is putted in the template folder, it will be used instead of the file in the Joom Donation component. So in the future, if you update Joom Donation, the your modifications will be not lost.
You should use overwrite output from the template in the case you just have some minor changes on a few of pages.