You can exit the inventory screen (or other screens) by pressing the same button you pressed to enter the inventory screen: Tab, Space, or Enter. Escape also works, but (as you noted) it is also used by Unity's web player to exit full screen mode.
The Steam version of the game has rebindable controls as well as a whole bunch of additional content. If you enjoy this version on itch, definitely check that out! (Funnily enough, the Escape key isn't rebindable on the Steam version either. It uses Unity's new-ish Input System, which allows live rebindings of keys - ya know, like "Press a key to rebind the Inventory Menu action". The problem is that you need to be able to cancel a live rebinding, even if you are rebinding the "cancel" action... After spending six weeks on the whole rebinding project, I took the short cut of always keeping Escape as the "cancel the rebinding" key. So it's the one key that can't be used for other things.)
THanks! I figured that out soon after, and I totally forgot to edit my response, but thanks!
Also, Is the steam verson free or is it over 20$? I'd be willing to pay over buth to support development and to get more content thatn whats featured here, but I'm kiinda hoping that it's -20
Great game! I'll change the rating (if not now, eventually..)
Glad you are enjoying it! The full game is $10 on Steam - you can find it at https://store.steampowered.com/app/1417750/Moondrop/. I'm most of the way through Early Access, with only two special Mountain Trails left to finish developing.