Is there a template for every theme?
No, and this is worth knowing before you search for one. There are dozens of designs; you pick the one whose colours and type read like your theme, then the dress code, running order and wording do the rest of the scene-setting.
Can I explain the dress code properly instead of one word?
Yes. The dress code has its own section on the page, which is where you say what the theme means in practice, what you have provided, and what happens to the guest who arrives in ordinary clothes.
Where do guests ask questions about the theme?
Birthday pages carry an FAQ section, so write the answers in advance: parking, children, food, costume. When a genuine question arrives that you had not thought of, add it there and everybody else sees it too.
Can I keep the theme from the guest of honour?
That is a matter of who you send the link to, plus a password on the page so it cannot be opened without it. Invite the collaborators who are in on it and leave the guest of honour off until the day.
How do we get photographs of the costumes afterwards?
Put the table code on the page and mention it in the invitation. Guests scan it and upload their own pictures as the evening goes on, and the gallery fills while the party is still happening.