Developing a theme is pretty safe. You won’t wreck anything. The minimum you need is style.css with the following header:
/*
Theme Name: example
*/
Then save your html file as index.php and add that too. If you have an html file and save it as a php file, it’ll be fine. If it contains no actual php code, it’ll just come out exactly the same. Then activate the theme, and the website will simply serve your index.php page every time. Not exactly what wordpress was designed to do, but no reason why that wouldn’t work.
That said, my recommendation would be to invest a little time learning some of the working parts with WordPress and, even perhaps in time, leveraging some of its actual potential. It’s a really, really great platform for a web developer to work from.
Edit:
Based on discussion in the comments, I think the actual solution to this will be to use a child theme along with one of two options for creating a template for a specific page.
First, a comment on child themes: The reason for this is so that you don’t have to make edits to the theme currently running. This will allow it to receive updates without deleting the changes you make. More on child themes here and here. Then, you can use either a globally accessible page template, discussed here, or (and this is the one I would recommend) a page template applied to just one page, as discussed here.
Make your child theme by making a new theme directory called whatever you want, containing an empty file called functions.php (You don’t have to use it, just have it here) and style.css with a header like this one:
/*
Theme Name: Example Child Theme
Template: existing-theme-directory-name
*/
Activate your child theme, and at this point nothing should change.
Then make a page in WordPress which you can leave blank, just publish it to get your URL. (take note of the page’s slug. If you miss it, an easy way to find it is click “quick edit” from the screen that shows all the pages) Then rename your HTML file page-my-page-slug.php (obviously using the page slug you collected earlier) and add it to your child theme directory. That file will now be served every time your page’s URL is requested.