The RSS reader has “CSS class” names set to various elements like title, description, thumbnail etc. Users can tweak the style using this as per their requirement. Below FAQ has the list of class names which you can use.
However, there is no form or GUI option provided to customize these considering the requirements are usually very diverse.
The ability to customize the RSS reader, the class names etc. remain the same in both free and PRO. The PRO version does not provide explicit options/form to customize that. You can continue with the free version if you are looking for customization alone. The list of PRO only features are listed below.
Super RSS Reader also provides WordPress “hooks” using which you can modify the output like including custom text, pulling custom feed data, adding custom class etc. Below document has more info on this.