getMuiThemeのカスタマイズ
http://www.material-ui.com/#/customization/themes
https://github.com/callemall/material-ui/blob/master/src/styles/getMuiTheme.js
https://github.com/callemall/material-ui/tree/master/src/styles
https://github.com/callemall/material-ui/blob/master/src/styles/baseThemes/darkBaseTheme.js
https://github.com/callemall/material-ui/blob/master/src/styles/baseThemes/lightBaseTheme.js