Theme not working on different browsers and computers

Hi,
I’m sorry if I’m being a complete and total idiot, I’ve only just started using wordpress and CSS and it’s still a big mystery to me.
I started editing my site using the sliding doors theme as a starting point, and previewed it in firefox. Everything seemed fine until I tested it on other computers and browsers – then a lot of my edits dissappeared. My website is at https://www.heartoftime.co.uk/home, and an image of what it’s supposed to look like is here https://i1067.photobucket.com/albums/u423/kairoscomic/a18ecf93.jpg.
The major issue is that the background messes up on most platforms. I suspect it has something to do with how I have edited the wrapper code, as it should be tight with the banner and menu. It should also have a paper texture image which is missing. There are a few other minor issues – such as the space in between the banner and the menu images getting bigger – but if these aren’t fixable I’m ok with that.
If you can spot any other errors please let me know. Again, I’m a complete novice so I apologize if I’ve made any blinding mistakes. I’ve tried searching for other people’s solutions on the forum but nothing seems to work, I’d be extremely grateful if you could offer some advice. Thank you!