where to change the align justify

Hello! thanks for this great theme!!! I really have this problem for my ignorance of css or html.

I want justify all the text in the content. I did it in html, but w3c says therea re many errorsI found the solution: doing it by css. I already change some parameters in the style.css file such as font style and size and others. But I couldnt find where to add {text-align:justify} for the contents...
Can anybody tell me if this parameter must be changed in the style.css file or where I need to look ?
GRACIAS!