hi Radu,
Really depends what you want to achieve.. Case you're going to use this everywhere (or many times in your application) you could extend the ResponsiveGridLayout (and the renderer) to re-implement where the main class is being written (Renderer.getMainClass - String) - add your own class there and use a custom css to code the style.
I would only go thru such if you really need to use this many times - this way you won't need to remember to always add a class into the view layout element - but if this is for one element or two I wouldn't even go thru the trouble of extending controls that could always introduce issues when patching to a higher major release.
Cheers,
Dan.