I'm an early stage founder who had the same question when I started.
Refactoring UI[0] is a book made by the folks at Tailwind Labs which opened my eyes to the basics of web design. It discusses in an accessible way the specific things your mention (font, color palette, basic layout, etc) and helps you build your intuition on those aspects of design.
I also recommend Tailwind UI[1], which is a software product made by the same people. Both of these resources were indispensable when I created Magistrate[2].
[0] https://www.refactoringui.com/
[1] https://tailwindui.com/
[2] https://getmagistrate.com/