Personally, I prefer to keep my logo in the html then make it responsive, or use media queries to adjust the size. See the codepen I created below. You can use a max-width or min-width if you feel it gets too large/small. I’m not a fan of using critical content as a CSS background. CSS should be styling only, keep the content (like logos) in the html.
It is pretty problematic that you don’t know what style sheet to add the code to. Is this a template where you can access all the code?
In a situation where you have no control over the all the files and tried every way to find it, you may need to override it with an !important (which I hate doing) but only if it is necessary.