feat: enable custom theme with eject

lib/commands/eject.js

@@ -0,0 +1,16 @@
+const fs = require('fs-extra');
+const chalk = require('chalk');
+const path = require('path');
+
+module.exports = async function eject(siteDir) {
+  const defaultTheme = path.resolve(__dirname, '..', 'theme');
+  const customTheme = path.resolve(siteDir, 'theme');
+  await fs.copy(defaultTheme, customTheme);
+
+  const relativeDir = path.relative(process.cwd(), customTheme);
+  console.log(
+    `\n${chalk.green('Success!')} Copied default theme files to ${chalk.cyan(
+      relativeDir
+    )}.\n`
+  );
+};