How to make the “big grid” element was had separate background?
or any part of the (new row in Visual Composer) had a different background color?
Different background in the video demo template:

Dark bg full with has “big grid” element
lighter bg has a different emenent with visual composer.
Where the admin template panel can I create new css styles?
Thank you for your answer.
Hello boos11,
If you want to achieve the similar layout for your homepage looks like the above example, you will have to use a single ROW in STRETCH mode with background color, like this -> http://screencast.com/t/UsQ3dbi7 -> http://screencast.com/t/b2dJk96q0yvp and then, please notice that you will have to use a bit of CSS code to align the Big Grid.
Thanks for your understanding!
Hello boos11,
The full-width background is given by the STRETCH ROW effect, as I explain you above3. Please ensure that you have followed all the steps from above in order to achieve as you like. Also, you can provide your website URL so I can inspect it closer.
Thanks for your understanding!
