check if reveal theme exists

lib/utils.js

@@ -1,4 +1,6 @@
 'use strict'
+const fs = require('fs')
+const path = require('path')
 
 exports.isSQLite = function isSQLite (sequelize) {
   return sequelize.options.dialect === 'sqlite'
@@ -23,3 +25,10 @@ exports.getImageMimeType = function getImageMimeType (imagePath) {
       return undefined
   }
 }
+
+exports.isRevealTheme = function isRevealTheme (theme) {
+  if (fs.existsSync(path.join(process.cwd(), 'public', 'build', 'reveal.js', 'css', 'theme', theme + '.css'))) {
+    return theme
+  }
+  return undefined
+}