Theme Fragments can be obtained either from searching random crates while on quests or as a reward for a quest itself. Each theme has 9 fragments that must be obtained before the theme can be crafted in the Theme Workshop. Once you have obtained all 9 fragments for all 8 themes and craft all of them, this achievement will unlock.
Pursue your 100 quests first. Many of your theme fragments will come from these quests. Once you have hit 100 quests, feel free to continue to do more quests to acquire more theme fragments (though at this point many of them will be duplicates), or do what I did, and spend Nuka-Cola to purchase the remaining fragments. Once you have them all, craft each of them. If you do what I recommended, you should be at a point in your game where you have caps and resources to spare, with time being the only real limiting factor for you.