My muffin builder is not working

I was trying to make my header image responsive on mobile so I add css.   On one safari my streched out and on firefox it seems to be fine.  

I made the header width:100% to see if that would help.   It didn't seem to help.   How can I make the header background image responsive? 


I was trying to change the custom css again to make it work but then muffin builder froze on me and it is not working. 

What can I do to fix this.  I just reinstalled the the whole wordpress theme.   Can you tell what I can do?  It's at isamplesite.com