improve theme resolution

lib/prepare.js

@@ -111,19 +111,26 @@ async function resolveOptions (sourceDir) {
     options.themePath = path.resolve(__dirname, 'default-theme/Layout.vue')
     options.notFoundPath = path.resolve(__dirname, 'default-theme/NotFound.vue')
   } else {
+    let themeDir
+    let themePath
     // resolve custom theme
-    const themeDir = siteConfig.theme
-      ? path.resolve(__dirname, `../../vuepress-theme-${siteConfig.theme}`)
-      : path.resolve(vuepressDir, 'theme')
-
-    const themePath = path.resolve(themeDir, 'Layout.vue')
-    if (fs.existsSync(themePath)) {
-      options.themePath = themePath
+    if (siteConfig.theme) {
+      try {
+        themePath = require.resolve(`vuepress-theme-${siteConfig.theme}/Layout.vue`)
+        themeDir = path.dirname(themePath)
+      } catch (e) {
+        throw new Error(`[vuepress] Failed to load custom theme "${
+          siteConfig.theme
+        }". File vuepress-theme-${siteConfig.theme}/Layout.vue does not exist.`)
+      }
     } else {
-      throw new Error(`[vuepress] Cannot resolve Layout.vue file for custom theme${
-        siteConfig.theme ? ` "${siteConfig.theme}"` : ``
-      }.`)
+      themeDir = path.resolve(vuepressDir, 'theme')
+      themePath = path.resolve(themeDir, 'Layout.vue')
+      if (!fs.existsSync(themePath)) {
+        throw new Error(`[vuepress] Cannot resolve Layout.vue file in .vuepress/theme.`)
+      }
     }
+    options.themePath = themePath
 
     const notFoundPath = path.resolve(themeDir, '/NotFound.vue')
     if (fs.existsSync(notFoundPath)) {