Looking at the screenshots and the CSS in question, and presuming that before it just wasn’t specifying fonts at all, I prefer the “before” result if I’m the consumer, mostly because I like the fonts I set in my browser, and consider Inter and Incosolata a slight regression. Also most of the other styles I disagree with; capping the width and reducing the line-height for headings is about all I actually agree with in there.
But really I think that if this is all you want to change with org-mode HTML output, it must export pretty decent HTML. (I’m not familiar with it.)
(Analysis of the things I particularly don’t like: 1.6 is too large a line-height; I prefer #000 to #444, though I’d settle for #222; image should use max-width instead of width, and have height: auto added, and what’s with the 700px instead of 900px, especially if you’re not going to block and centre it?; border-radius on images seems a bad idea; I dislike gratuitous fiddling with ::selection colours—only touch it if you have actual brand colours that you’re making it align with, like https://wesbos.com/, and be cautious even in that, as is demonstrated in the .highlight on that page which thwarts selection styling.)
Org mode html export is clean by default, yet customizable.
I like that the outline is represented in the dom as nested elements instead of a flat representation.
Title, status, tags, properties, description, code and code output are well separated and easy to select through path, I’d and/or class.
Something I enjoy is the "narrow to sub tree" feature that allows to export a nice html rendered export from a part of the document.
I have been using that feature to keep all my uni assignments in the same file. Although I export to LaTeX and not html. Though I can export it as a single large webpage with all the assignments on, and it mostly work.
But really I think that if this is all you want to change with org-mode HTML output, it must export pretty decent HTML. (I’m not familiar with it.)
(Analysis of the things I particularly don’t like: 1.6 is too large a line-height; I prefer #000 to #444, though I’d settle for #222; image should use max-width instead of width, and have height: auto added, and what’s with the 700px instead of 900px, especially if you’re not going to block and centre it?; border-radius on images seems a bad idea; I dislike gratuitous fiddling with ::selection colours—only touch it if you have actual brand colours that you’re making it align with, like https://wesbos.com/, and be cautious even in that, as is demonstrated in the .highlight on that page which thwarts selection styling.)