How to create a html component with fixed width

Look at these pictures. I have changed the display size but the website width doesn't change (remains 960 px). I want to copy the Tumblr first page with exact sizes and fonts. I need to know how to create a webpage with a fixed size.

I've checked the CSS files, they have used 'px' unit for width. However, I know that the 'px' unit is an absolute unit and it is not a good choice for a responsive webpage.