I would take it easy, frankly! Little kids couldn't care less about a theme, and they won't notice it or appreciate it! You'll do all kinds of work, exhaust yourself, and no one will care!
Decorate with purple if you think your daughter will notice. Cupcakes are the rage anyway, so you can do that if you want. Some people let the kids decorate their own cupcakes with little candies & icing. 3 year olds will need help but it's an easy activity.
Kids also don't care about a themed favor - they just like a goody bag or a toy.
I'm a former teacher too - and I know how overstimulated kids can get if adults have too much planned for them. Resist that teacher urge to develop a full lesson plan! LOL We always had very simple and small parties, and only invited my son's very good friends. When he was 5, he invited 5 friends. When he was 6, he had 6. Other parents thanked us for keeping it reasonable and always complained about huge parties - if everyone gets into reciprocating, there's a party (and a gift purchase) every weekend.
If your daughter likes books, then consider buying each child a small book. Or just do a small simple craft (foam shapes on a picture frame?) - then the child can take that home as their "favor" and use it for a photo frame or to display their own art work. They can glue a magnetic strip to the back and it will stick on the fridge.