Can edit, but not update themes and stylesheet

I’ve been spending a lot of time reading about how to change code for various reasons under ‘appearance; editor,’ and I understand how to get there and use the links on the right to find the right section of code. The problem is, every time I make changes and then hit the “update file” button, it reverts back to what it was originally.

Someone else created the site, and I am now editing it. I’m not trying to change style (for the most part, although going into the style sheet and making the body text bigger is also not working, for the same reason), just content that someone else originally entered. I’ve checked permissions and my computer is the “owner” and has total permission for everything from what I can tell. There’s also no message that “this would be writable if you had permission to write it” or anything to indicate I’m going about it wrong. It just updates/refreshes the page and my changes have simply reverted back to what they were before.

Clearly this is going to be simple to fix because there’s no way not being able to edit these things is a common problem – that just wouldn’t fly. But I’ve seriously spent a lot of time trying to figure it out by reading past forum pasts and other help documents, and I haven’t found anything that works yet. So help, please.

http://www.bgcflag.org