docs: update description of config.theme

Author: ulivz

packages/docs/docs/config/README.md

@@ -159,7 +159,11 @@ VuePress provides a convenient way to add extra styles. you can create an `.vuep
 - Type: `string`
 - Default: `undefined`
 
-Specify this to use a custom theme. With the value of `"foo"`, VuePress will attempt to load the theme component at `node_modules/vuepress-theme-foo/Layout.vue`.
+Specify this to use a custom theme.
+
+**Also see:**
+
+- [Using a theme](../theme/README.md#using-a-theme).
 
 ### themeConfig
 

packages/docs/docs/zh/config/README.md

@@ -154,7 +154,11 @@ VuePress provides a convenient way to add extra styles. you can create an `.vuep
 - 类型: `string`
 - 默认值: `undefined`
 
-当你使用自定义主题的时候，需要指定它。当值为 `"foo"` 时，VuePress 将会尝试去加载位于 `node_modules/vuepress-theme-foo/Layout.vue` 的主题组件。
+当你使用自定义主题的时候，需要指定它。
+
+**参考:**
+
+- [Using a theme](../theme/README.md#using-a-theme).
 
 ### themeConfig