Only add loader if there is a css-loader

Author: silvenon

packages/gatsby-plugin-css-customs/gatsby-node.js

@@ -1,12 +1,15 @@
 module.exports.onCreateWebpackConfig = (
-  { actions, getConfig, rules },
+  { actions, getConfig, loaders, rules },
   pluginOptions
 ) => {
   const { cssModules = false } = pluginOptions
 
   const config = getConfig()
+  const cssLoader = loaders.css()
   const plainCssRule = rules.css()
   const cssModulesRule = rules.cssModules()
+
+  const isCssLoader = ({ loader }) => loader === cssLoader.loader
   const isPlainCssRule = rule => (rule.test.source = plainCssRule.test.source)
   const isCssModulesRule = rule =>
     (rule.test.source = cssModulesRule.test.source)
@@ -19,10 +22,10 @@ module.exports.onCreateWebpackConfig = (
   cssRules.oneOf
     .filter(cssModules ? isAnyCssRule : isPlainCssRule)
     .forEach(rule => {
-      const insertIndex = rule.use.findIndex(({ loader }) =>
-        loader.match(/\bcss-loader\b/)
-      )
-      rule.use.splice(insertIndex, 0, cssCustomsLoader)
+      const insertIndex = rule.use.findIndex(isCssLoader)
+      if (insertIndex !== -1) {
+        rule.use.splice(insertIndex, 0, cssCustomsLoader)
+      }
     })
 
   actions.replaceWebpackConfig(config)