Customization for Group Profile Page

I want to add some customized HTML and CSS to the group profile page. For example, adding some icons or CSS properties to widget header like "Group activity" then followed by icon.  What is the best way to achieve this? I try not to avoid using existing plugins as possible. 

Thanks!

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking