Remove style override limitation to default theme

Closes vuejs#404.

Author: ghys

lib/prepare.js

@@ -61,7 +61,7 @@ if (!Object.assign) Object.assign = require('object-assign')`
 
   // 5. handle user override
   const overridePath = path.resolve(sourceDir, '.vuepress/override.styl')
-  const hasUserOverride = options.useDefaultTheme && fs.existsSync(overridePath)
+  const hasUserOverride = fs.existsSync(overridePath)
   await writeTemp(`override.styl`, hasUserOverride ? `@import(${JSON.stringify(overridePath)})` : ``)
 
   // 6. handle enhanceApp.js