The single biggest change anyone can make to their site is sorting out the typography. There are plenty of freely available typefaces out there. My favourites are IBM Plex (Sans Serif, Serif and Mono), Inter (Sans Serif), Merriweather (Sans Serif and Serif) and Source Sans and Source Serif, Charter (Serif) & Clear Sans (Sans Serif).
The issue is that because they are freely available, they offer little distinction as they can be used by everyone. Sometimes that’s what you want. Sometimes it’s not. If I was running an up and coming blogging platform (ahem @manton), I’d try and hook up with indie typographers. People like Matthew Butterick, Mika Melvas or even a foundary like TipoType. See if I can strike some kind of deal to allow everyone to use their fonts across my platform.