This is my place: https://www.airbnb.com/rooms/52612233
When we bought it, it was completely unthemed, but furnished.
Managing about a dozen properties in the area (and owning one) I would say one thing that I think relates to your question before I get into the costs. If you're working with a budget it's going to go a lot further in terms of your returns if you get a place already furnished and then just throw out what you don't need or what you end up theming, rather than starting with nothing a spending a fortune on stuff that's not going to matter very much to your bottom line.
In other markets and with other approaches, design of regular every day items can really matter and help a place stand out. Here, if you're going with the theming approach, that theming is all people are going to care about.
To put it another way, if people are deciding between a bowling alley or a golf simulator or a $10k arcade machine or a theater or an X-Wing, the dresser in the 8th bedroom or the coffee table in the 3rd living space aren't going to drive their decision. But these houses are huge and furnishing them out is very expensive. I've seen people in the area buy new construction unfurnished and spend $100k+ just to furnish them. You are going to get a WAY better return on that $100k by buying an already furnished place and spending $100k on awesome theming.
Now, with that out of the way, regarding our place and our experience/costs theming it...
We didn't hire a "do it all" themer/designer. We kind of planned it ourselves and hired subs. It was a massive headache dealing with some of them, though we did save money in the end.
Garage Arcade - We hired someone to drywall in the garage door, build a closet around the water heater, paint, and lay carpet. Hired someone else to do the electrical and help us source the arcade games. Total cost was around $20k.
Star Wars room - This was the spot we hired a designer for, though it was just for this room and not the whole house. We told her generally what we wanted (X-wing vs. Tie Fighter with a bridge connecting them) and let her run with it. This is an oversized 10x20 room. Cost was $18k.
Princess room - I actually designed this one myself. The bed I was actually able to find, and then we did the room around it. The wall is just wallpaper we got off etsy, and Disney princess fathead stickers. We hired someone to assemble the bed, hired someone to put up the wallpaper, hired someone to run the LED lights and hang the chandelier. Total cost was a little over $3k.
We spent another $14k having someone fill out the rest of the house including the Toddler clubhouse under the stairs, the "lightly" themed rooms like Toy Story and 101 Dalmations, and just general decor and furniture that wasn't already in place. We could have done this for much less ourselves, but at the time we were very busy and just didn't have the time to do it ourselves.
So all-in after we added coffee bar, kitchen item, etc (the place was managed by Vacasa before we bought it so it was very lacking in things an STR needs) we were around $60k, not counting a few repairs we had to do on the driveway/pool/etc.
Had we hired a do-it-all designer and not had to manage the subs ourselves it would have been a LOT less of a headache, but probably would've hit $100k easily. Were the home unfurnished when we started, at least $150k.
If you really want to go all out I've seen designers in the $300k range do homes that make mine look like a piece of junk. I manage one of these, and it's an awesome place. Those are unfortunately getting more and more common now as the barometer of what constitutes a super property continues to rise. 3 years ago I'd have said my property was 98th percentile in the area. Now I'd put it around 75th percentile.