Fix [object Object] is not a PostCSS plugin

package.json

@@ -56,6 +56,7 @@
         "gulp-util": "3.0.8",
         "gulp-watch": "5.0.1",
         "gulp-zip": "5.1.0",
+        "postcss": "8.2.14",
         "postcss-color-mod-function": "3.0.3",
         "postcss-easy-import": "3.0.0"
     },

yarn.lock

@@ -3522,6 +3522,11 @@ nan@^2.12.1, nan@^2.14.0:
   resolved "https://registry.yarnpkg.com/nan/-/nan-2.14.2.tgz#f5376400695168f4cc694ac9393d0c9585eeea19"
   integrity sha512-M2ufzIiINKCuDfBSAUr1vWQ+vuVcA9kqx8JJUsbQi6yf1uGRyb7HfpdfUr5qLXf3B/t8dPvcjhKMmlfnP47EzQ==
 
+nanoid@^3.1.22:
+  version "3.1.22"
+  resolved "https://registry.yarnpkg.com/nanoid/-/nanoid-3.1.22.tgz#b35f8fb7d151990a8aebd5aa5015c03cf726f844"
+  integrity sha512-/2ZUaJX2ANuLtTvqTlgqBQNJoQO398KyJgZloL0PZkC0dpysjncRUPsFe3DUPzz/y3h+u7C46np8RMuvF3jsSQ==
+
 nanomatch@^1.2.9:
   version "1.2.13"
   resolved "https://registry.yarnpkg.com/nanomatch/-/nanomatch-1.2.13.tgz#b87a8aa4fc0de8fe6be88895b38983ff265bd119"
@@ -4229,6 +4234,15 @@ postcss-values-parser@^2.0.0:
     indexes-of "^1.0.1"
     uniq "^1.0.1"
 
+postcss@8.2.14:
+  version "8.2.14"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-8.2.14.tgz#dcf313eb8247b3ce8078d048c0e8262ca565ad2b"
+  integrity sha512-+jD0ZijcvyCqPQo/m/CW0UcARpdFylq04of+Q7RKX6f/Tu+dvpUI/9Sp81+i6/vJThnOBX09Quw0ZLOVwpzX3w==
+  dependencies:
+    colorette "^1.2.2"
+    nanoid "^3.1.22"
+    source-map "^0.6.1"
+
 postcss@^6.0.1, postcss@^6.0.11:
   version "6.0.23"
   resolved "https://registry.yarnpkg.com/postcss/-/postcss-6.0.23.tgz#61c82cc328ac60e677645f979054eb98bc0e3324"