fix($core): cannot retrieve the correct theme name when them path is …

…a local absolute path linked to a javascript file.

packages/@vuepress/core/lib/node/loadTheme.js

@@ -32,11 +32,15 @@ const ThemeAPI = require('./theme-api')
 
 module.exports = function loadTheme (ctx) {
   const themeResolver = getThemeResolver()
-
   const theme = resolveTheme(ctx, themeResolver)
+
   if (!theme.path) {
-    throw new Error(`[vuepress] You must specify a theme, or create a local custom theme. \n For more details, refer to https://vuepress.vuejs.org/guide/custom-themes.html#custom-themes. \n`)
+    throw new Error(
+      '[vuepress] You must specify a theme, or create a local custom theme. \n'
+      + 'For more details, refer to https://vuepress.vuejs.org/guide/custom-themes.html#custom-themes. \n'
+    )
   }
+
   let applyTip = `Apply theme ${chalk.magenta(theme.name)}`
   theme.entry.name = '@vuepress/internal-theme-entry-file'
 
@@ -46,6 +50,7 @@ module.exports = function loadTheme (ctx) {
     parentTheme.entry.name = '@vuepress/internal-parent-theme-entry-file'
     applyTip += chalk.gray(` (extends ${chalk.magenta(parentTheme.name)})`)
   }
+
   logger.tip(applyTip + ' ...')
 
   logger.debug('theme', theme.name, theme.path)
@@ -74,7 +79,9 @@ function resolveTheme (ctx, resolver, ignoreLocal, theme) {
   let shortcut
   let entry = {}
 
-  // 1. From local
+  /**
+   * 1. From `.vuepress/theme` directory.
+   */
   if (!ignoreLocal
     && !fs.existsSync(theme)
     && fs.existsSync(localThemePath)
@@ -84,15 +91,31 @@ function resolveTheme (ctx, resolver, ignoreLocal, theme) {
     name = shortcut = 'local'
     logger.tip(`Apply local theme at ${chalk.gray(path)}...`)
 
-    // 2. From dep
+    /**
+     * 2. From deps or custom local path.
+     *    - vuepress-plugin-foo
+     *    - /path/to/a-theme/index.js
+     */
   } else if (isString(theme)) {
+    /**
+     * To let theme resolver get the correct theme name.
+     */
+    if (theme.endsWith('/index.js')) {
+      theme = theme.replace(/\/index\.js$/, '')
+    }
+
     const resolved = resolver.resolve(theme, sourceDir)
     if (resolved.entry === null) {
       throw new Error(`Cannot resolve theme: ${theme}.`)
     }
+
     path = normalizeThemePath(resolved)
     name = resolved.name
     shortcut = resolved.shortcut
+
+    /**
+     * 3. fallback
+     */
   } else {
     return {}
   }