Problem with custom character sheet?

So I inserted a custom character sheet made by another person in my homebrew game, and it worked fine. However, when I make some minor changes CSS and HTML in font size and type, and rename a few things in the settings of my game, these changes don't stick. I save the  changes and they show up in the preview, but in the game they don't show in existing character sheets nor new ones. Am I not supposed to edit the character sheet template in any way, after I stick it in my game? My first chnage of font did stick, but after that no changes seem to apply. What happens if I delete the character sheet template, does it affect my existing character, or can I try to re-import the sheet template? Should I make a copy of the game before attempting anything - I read you cant make backups of characters in any other way?

It's difficult to offer advice without a more complete description of what is happening. What code did you change, what was the before and after (not the whole sheet, just the specific change you made). Also what do you mean by "character sheet template" - there's a lot of things that could mean in roll20. Generally its a good idea to make a copy of a game before making any major changes, yes.

I guess what I am referring to is a custom character sheet which I added to my game in the settings. So I copied the html, CSS, and json from github. It worked alright, but I wasn't happy with some little details. All I did was change some CSS and HTML which define font family and size for layout reasons, and then one title on the sheet (because it was too long and kinda messing the layout). I didnt touch any js stuff or variables or anything. The text on the sheet was too large and hard to read due to the font used, so I was attempting to make it smaller and clearer. Very basic stuff. I managed to change to font, but that was it. The actual title doesnt' change in my game character sheets, even if it shows changed on the preview sheet. I tried to be careful and not remove or change anything else, but I guess I could just start all over again? What happens if I copy the code again to game settings / character sheet template, will it bork my current characters in the game?

We cant help you without seeing what the code you changed looks like. Both what it was originally, and what you changed it to. And I stress again, we only need to see the relevant parts - not the whole sheet.  There's so many ways you can change the code, and so many ways it could break or have no effect, that it's impossible to guess what's happening without seeing the code.