Improve styles for mobile devices

[selfthinker]

[lpaulger]

I've added support for pixel-ratios in the scss version (part of the pull request)
https://github.com/selfthinker/CSS-Playing-Cards/pull/6/files#diff-f2eb329f0de5ccd5fd6d93891db21df9R542

and I'm using this lib on mobile devices, check out my changes :)