config theme.json

themes.json

{
  "backend": {
    "src"    : "app/design/adminhtml/Magento/backend",
    "dest"   : "pub/static/adminhtml/Magento/backend",
    "locale" : ["en_US"],
    "lang"   : "less",
    "default": true,
    "area"   : "adminhtml",
    "vendor" : "Magento",
    "name"   : "backend",
    "files"  : [
      "css/styles-old",
      "css/styles"
    ]
  },
  "blank": {
    "src"    : "app/design/frontend/Magento/blank",
    "dest"   : "pub/static/frontend/Magento/blank",
    "locale" : ["en_US"],
    "lang"   : "less",
    "default": true,
    "area"   : "frontend",
    "vendor" : "Magento",
    "name"   : "blank",
    "files"  : [
      "css/styles-m",
      "css/styles-l",
      "css/email",
      "css/email-inline"
    ]
  },
  "luma": {
    "src"    : "app/design/frontend/Magento/luma",
    "dest"   : "pub/static/frontend/Magento/luma",
    "locale" : ["en_US"],
    "lang"   : "less",
    "default": true,
    "area"   : "frontend",
    "vendor" : "Magento",
    "name"   : "luma",
    "files"  : [
      "css/styles-m",
      "css/styles-l"
    ]
  },
  "sass-blank": {
    "src"    : "app/design/frontend/Gold/Blank",
    "dest"   : "pub/static/frontend/Gold/Blank",
    "locale" : ["en_US"],
    "lang"   : "scss",
    "postcss": ["plugins.autoprefixer()"]
  }
//  "sass-custom": {
//    "src"    : "vendor/snowdog/theme-custom",
//    "dest"   : "pub/static/frontend/snowdog/custom",
//    "parent" : "sass-blank",
//    "locale" : ["en_US", "pl_PL"],
//    "lang"   : "scss",
//    "postcss": ["plugins.autoprefixer()"]
//  }
}