The theme is that true happiness comes from appreciating what we already have.
True happiness comes from within, and not from material things such as money or other worldly possessions. If you can be happy without such things, you will achieve true happiness.
Explanation:
The 3 brass pennies", base on my analysis, the theme of the story is obtaining what you want doesn't guarantee happiness.
I hope you're happy with my answer and be happy to invite additional if you have got queries and more clarifications
The story shows that obtaining knowledge, foresight, material resource and such alternative things doesn't offer one true happiness. True happiness comes from inside ourselves.
