Update rollup and postcss config

Author: spinn

Diff for: build/rollup.config.js

@@ -179,17 +179,17 @@ export default [
       copy({
         targets: [
           {
-            src: 'examples/questionnaire/branding.css',
+            src: 'src/assets/css/themes/theme-minimal.css',
             dest: 'dist',
             rename: 'vue-flow-form.theme-minimal.css'
           },
           {
-            src: 'src/assets/css/themes/green-theme.css',
+            src: 'src/assets/css/themes/theme-green.css',
             dest: 'dist',
             rename: 'vue-flow-form.theme-green.css'
           },
           {
-            src: 'src/assets/css/themes/purple-theme.css',
+            src: 'src/assets/css/themes/theme-purple.css',
             dest: 'dist',
             rename: 'vue-flow-form.theme-purple.css'
           }

Diff for: package-lock.json

@@ -36,9 +36,9 @@
   ],
   "scripts": {
     "serve": "vue-cli-service serve",
-    "build": "vue-cli-service build && npm run cssmin",
-    "build:npm": "rollup --config build/rollup.config.js",
-    "cssmin": "postcss dist/vue-flow-form.theme-minimal.css > dist/vue-flow-form.theme-minimal.min.css && postcss dist/vue-flow-form.theme-green.css > dist/vue-flow-form.theme-green.min.css && dist/vue-flow-form.theme-purple.css > dist/vue-flow-form.theme-purple.min.css"
+    "build": "vue-cli-service build",
+    "build:npm": "rollup --config build/rollup.config.js && npm run cssmin",
+    "cssmin": "postcss dist/vue-flow-form.theme-minimal.css -o dist/vue-flow-form.theme-minimal.min.css && postcss dist/vue-flow-form.theme-green.css -o dist/vue-flow-form.theme-green.min.css && postcss dist/vue-flow-form.theme-purple.css -o dist/vue-flow-form.theme-purple.min.css"
   },
   "eslintConfig": {
     "root": true,
@@ -64,6 +64,8 @@
     "babel-helper-vue-jsx-merge-props": "^2.0.2",
     "babel-plugin-syntax-jsx": "^6.18.0",
     "babel-plugin-transform-vue-jsx": "^3.3.0",
+    "postcss": "^7.0.32",
+    "postcss-cli": "^7.1.2",
     "postcss-import": "^12.0.1",
     "prettier-eslint": "^8.8.2",
     "rollup": "^1.17.0",