Embed a responsive google maps

Dear all,
I am a newbie in wordpress. Here is my request :

I currently use a them in wordpress which is fully responsive.
I want to add a google maps from google maps engine lite. My big problem is that I don’t know how to make it responsive.

I found some interesting stuff on the internet.

For example this link : link :

I need to add a css code in the theme but I don’t know where. In the style.css?

Then, they say adding the HTML code in a new page it would work perfectly.

Any ideas?