100% understood, and thanks for the follow-up. Perhaps I wasn't clear enough previously. @Jimako's initial reply was enough to help me undertand that I needed to make my changes only via CSS, but his reference to head_default.php (and a quick read through that file) was the thing that helped me understand that the CSS files get dynamically called sort of 'on the fly' by PHP (not HTML).
By the way, I embarked on all of this because the theme I have chosen is specifically coded to use five distinct fonts (Lato, Monserrat, Open Sans, Playfair Display and Raleway) that were not actually installed when I installed the website.
With your help, I have now:
- Downloaded and added all five webfonts into a new \fonts sub-directory of the theme directory.
- Added their required @font-face definitions at the very top of the style.css that already existed in the theme directory.
I'm pleased to say it is all working beautifully.
I will keep a backup of the amended style.css, along with some file notes, in case any future update messes with the default style.css.