fix($core): Index.styl is not injected at the end of the style bundle (close: #1523)

BTW, A theme cannot require the theme's styles at the layout but need use `Index.styl`  instead.

packages/@vuepress/core/lib/node/internal-plugins/palette/index.js

@@ -31,15 +31,22 @@ module.exports = (options, ctx) => ({
       ? `@import(${JSON.stringify(userPalette.replace(/[\\]+/g, '/'))})`
       : ''
 
+    const nullComment = '// null'
+
     // user's palette can override theme's palette.
-    let paletteContent = themePaletteContent + userPaletteContent
+    let paletteContent = '// Theme\'s Palette\n'
+      + (themePaletteContent || nullComment)
+      + '\n\n// User\'s Palette\n'
+      + (userPaletteContent || nullComment)
 
     if (ctx.themeAPI.existsParentTheme) {
       const parentThemePalette = path.resolve(ctx.themeAPI.parentTheme.path, 'styles/palette.styl')
       const parentThemePaletteContent = fs.existsSync(parentThemePalette)
         ? `@import(${JSON.stringify(parentThemePalette.replace(/[\\]+/g, '/'))})`
         : ''
-      paletteContent = parentThemePaletteContent + paletteContent
+      paletteContent = '// Parent Theme\'s Palette\n'
+        + (parentThemePaletteContent || nullComment)
+        + '\n\n' + paletteContent
     }
 
     await writeTemp('palette.styl', paletteContent)

packages/@vuepress/core/lib/node/internal-plugins/style/index.js

@@ -30,14 +30,23 @@ module.exports = (options, ctx) => ({
       ? `@import(${JSON.stringify(userStyle.replace(/[\\]+/g, '/'))})`
       : ''
 
-    let styleContent = themeStyleContent + userStyleContent
+    const nullComment = '// null'
+
+    // user's styles can override theme's styles.
+    let styleContent = '// Theme\'s Styles\n'
+      + (themeStyleContent || nullComment)
+      + '\n\n// User\'s Styles\n'
+      + (userStyleContent || nullComment)
 
     if (themeAPI.existsParentTheme) {
       const parentThemeStyle = path.resolve(themeAPI.parentTheme.path, 'styles/index.styl')
       const parentThemeStyleContent = fs.existsSync(parentThemeStyle)
         ? `@import(${JSON.stringify(parentThemeStyle.replace(/[\\]+/g, '/'))})`
         : ''
-      styleContent = parentThemeStyleContent + styleContent
+
+      styleContent = '// Parent Theme\'s Styles\n'
+        + (parentThemeStyleContent || nullComment)
+        + '\n\n' + styleContent
     }
 
     await writeTemp('style.styl', styleContent)

packages/@vuepress/theme-default/layouts/404.vue

@@ -24,5 +24,3 @@ export default {
   }
 }
 </script>
-
-<style src="../styles/theme.styl" lang="stylus"></style>

packages/@vuepress/theme-default/layouts/Layout.vue

@@ -147,4 +147,3 @@ export default {
 </script>
 
 <style src="prismjs/themes/prism-tomorrow.css"></style>
-<style src="../styles/theme.styl" lang="stylus"></style>