refactor: load theme

lib/load/theme.js

@@ -0,0 +1,20 @@
+const fs = require('fs-extra');
+const path = require('path');
+
+module.exports = function loadConfig(siteDir) {
+  const customThemePath = path.resolve(siteDir, 'theme');
+  const themePath = fs.existsSync(customThemePath)
+    ? customThemePath
+    : path.resolve(__dirname, '../theme');
+
+  const themeComponents = ['Docs', 'Loading', 'NotFound'];
+  themeComponents.forEach(component => {
+    if (!require.resolve(path.join(themePath, component))) {
+      throw new Error(
+        `Failed to load ${themePath}/${component}. It does not exist.`
+      );
+    }
+  });
+
+  return themePath;
+};