A themed place or event is one that has been deliberately decorated and run so that everything in it matches one idea, and being inside it feels like being somewhere else — another country, another time, a story world. A themed restaurant might have all the waiters dressed as pirates; a themed hotel room might be built to look like a spaceship; a 1920s-themed party asks guests to come in 1920s clothes. The key is that the theme is put on ON PURPOSE and applies to everything — the food, the music, the clothes, the walls.

It is very commonly used as the second half of a compound with a hyphen: horror-themed, ocean-themed, Christmas-themed, football-themed, Halloween-themed. That pattern is enormously productive — you can make a new one whenever you like and English speakers will understand it instantly. On its own, 'a themed event', 'a themed bar', it is often shorthand for the same thing.

It is also the past form of the verb 'to theme' ('they themed the whole wedding around the sea'), but honestly the adjective is what you will meet ninety-nine times out of a hundred.

One small piece of flavour: to many English speakers 'themed' carries a faint smile — it hints at something commercial, a bit over-the-top, made for tourists or children. A 'themed pub' is not a compliment in Britain. It is not a rude word at all, but it rarely describes something elegant. Note too that this everyday 'costume and decoration' sense sits some way from the serious 'theme' of a novel — nobody calls a sad literary novel 'themed'.