Edit 18/6/17: Gamemode: Wordament (Also Daily Challanges)
Theme words are the words that are part of the 'Special Challange / Theme' of that level. (For example: ''Use B/U to create words) These words apear as blue in your word list. You need a total of 50 'Special / Themed words' to unlock the achievement. The achievement will pop when you finish the level.