set defaultTheme by specifying theme-name

[silveltman]

I think it would be cool do do something like this:

defaultTheme: 'light',
themes: [
{
	name: 'light',
	extend: {
		colors: {
			base: getRadixColors(radixColors.gray)
		}
	}
},
{
	name: 'dark',
	extend: {
		colors: {
			base: getRadixColors(radixColors.grayDark)
		}
	}
},

Related to #73 , but a different approach

[RyanClementsHax]

Just to make sure I understand, you're specifying to use the defaultTheme value to specify which of the themes in the themes array should be considered default?