My Best Explanation:
A ROM is like a dog. All dogs are different lengths and sizes. A theme is like a silly sweater that grandma knits for the dog. Now, each sweater needs to be knit to fit the dog it was made for. If you try to put the big sweater on the small dog, it might fit but it wont work quite right... and if you try to put the small sweater on the big fat dog, it won't work at all!