Hard time understanding how to use the Style panel

I can’t seem to change any of the elements css on the page the panel shows the elements css but no matter where i look or what I do I can’t understand how to make any changes.

I have also attached my own new style sheet and can’t add to that file either.

What am I doing wrong, this should not be this difficult.I have tried to follow the How to use style panel but that never seem to help either