Using Appearance > Editor to change code is risky. That way lies panic and disaster if something goes wrong. A better way to do it is to install Filezilla and connect to your site using FTP to make changes. Yes, something else to learn. But it will soon make your life much easier.
Make a backup of your entire site. Then it doesn’t matter what you do, you can safely change anything but still revert back to how it looked before. There are plugins to help you make backups but it’s very important that they backup everything (files, data, uploaded images etc).
Removing formatting (as per Hannah’s suggestion) is a great idea. It shouldn’t foul anything up. There shouldn’t be any CSS formatting in the html anyway, just plain simple html, so removing it all in one fell swoop shouldn’t need to worry you! All the time you spent writing content (the most important thing) will not be wasted, that won’t get erased.
The way your site is now, adding more code to it and making minor changes will probably make things worse.
To change CSS, you need to edit the style.css file but not html files and not pages and posts in WordPress.
For example, one single addition to the end of the style.css file such as
`p {
color: pink;
}’
would turn all of your paragraph text pink. So little code, such big impact. That’s why I suggested you revert to the original out-of-the-box theme then edit its CSS.
A couple of dozen little code changes, and you’ll have a smart looking branded website in no time at all.
But BACKUP and CHILD THEME first.