support color-scheme

• add new option for theme to the config in order to understand if site supports auto switch between light and dart theme
• add color-scheme: light dark when the param is true, add light or dark when false and switch coloe-scheme value accordingly
• remove this change https://github.com/umputun/remark42/pull/1429

Cases

Auto

var remark_config = {
  theme: 'auto'
}

color-scheme: light dark;

Add support for auto switch between themes

Dark

var remark_config = {
  theme: 'dark'
}

color-scheme: dark;

Light

var remark_config = {
  theme: 'light'
}

color-scheme: light;