Really? In order to get the different font weights...

You have to specify them in the link URL:

1. @import url(https://fonts.googleapis.com/css?family=Source+Sans+Pro:100,200,300,400,500,600,700,800,900);  
2. 
   
3. body {  
4.     font-family: 'Source Sans Pro', serif;  
5.     font-size: 14px;  
6. }  
7.   
8. .wiki-content strong {  
9.  font-weight: 900;  
10. }  

Great Google configurator page to help you understand how to customize right weights and get that into the URL.