It appears that the repository has undergone several changes and renamings:

backend/node_modules/postcss/lib/declaration.js

@@ -1,24 +0,0 @@
-'use strict'
-
-let Node = require('./node')
-
-class Declaration extends Node {
-  get variable() {
-    return this.prop.startsWith('--') || this.prop[0] === '$'
-  }
-
-  constructor(defaults) {
-    if (
-      defaults &&
-      typeof defaults.value !== 'undefined' &&
-      typeof defaults.value !== 'string'
-    ) {
-      defaults = { ...defaults, value: String(defaults.value) }
-    }
-    super(defaults)
-    this.type = 'decl'
-  }
-}
-
-module.exports = Declaration
-Declaration.default = Declaration