[Grid] Add root class

[eps1lon]

Brings the CSS API in line with the other components. Allows for theme wide style customization of any Grid component.

[mui-pr-bot]

Details of bundle changes.

Comparing: 240dc18...afdfcc5

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	349,839	349,854	89,879	89,880
@material-ui/core/Paper	0.00%	0.00%	67,853	67,853	19,820	19,820
@material-ui/core/Paper.esm	0.00%	0.00%	60,184	60,184	18,563	18,563
@material-ui/core/Popper	0.00%	0.00%	30,463	30,463	10,530	10,530
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,364	17,364	5,726	5,726
@material-ui/core/useMediaQuery	0.00%	0.00%	2,469	2,469	1,041	1,041
@material-ui/lab	0.00%	0.00%	147,758	147,758	43,621	43,621
@material-ui/styles	0.00%	0.00%	53,105	53,105	15,433	15,433
@material-ui/system	0.00%	0.00%	17,136	17,136	4,525	4,525
Button	0.00%	0.00%	87,927	87,927	26,054	26,054
Modal	0.00%	0.00%	82,035	82,035	24,552	24,552
colorManipulator	0.00%	0.00%	3,232	3,232	1,299	1,299
docs.landing	+0.03% 🔺	+0.11% 🔺	49,899	49,914	10,803	10,815
docs.main	0.00%	0.00%	645,314	645,314	200,459	200,459
packages/material-ui/build/umd/material-ui.production.min.js	+0.01% 🔺	-0.00%	298,242	298,257	82,697	82,695

Generated by 🚫 dangerJS against afdfcc5