If you can create a simple sample that shows (has) the problem then you will get an answer. It is likely that anyone else that wants to help will also request additional information, especially source code, and we really prefer to have just the minimum source code (or procedure) that recreates the problem.
I posted feedback for Introduction to Razor Pages in ASP.NET Core and it was immediately closed; see Styling · Issue #24708 · dotnet/AspNetCore.Docs.
And changing the font-size in the site.css file is not enough; I need to change (reset) the font (font family) and that is not easy to determine how. I sure do not see how to do that in the site.css file.
No, nothing yet has accomplished what I wanted. I should have clarified my question so that it asks about the application that VS generates. I find no relevant documentation of that. I thought that changing the font size would be enough but it is not. I eventually decided to just remove Bootstrap from the application; that was easier to do than expected. The relevant Microsoft personnel think they do not need to provide any assistance and I disagree so that makes it difficult to decide what the answer is.
Thank you. I think it would be relevant for the description of the ASP.Net Core Web App to say something about the use of Bootstrap.
Perhaps it is as simple as using revert but I am not sure where Bootstrap intends us to put that. Unless someone says otherwise I will use the relevant Bootstrap documentation.
Thank you. I did consider submitting feedback and it helps to get verification that that would be appropriate.
I did just now discover that if I add font-size: 14px; to the html style in the wwwroot\css\site.css file then that does reduce the font size. The article that helped me is What Are the Bootstrap 4 Text Font Sizes and How Do You Change Them? 2022 - Bootstrap Creative. Probably people experienced with Bootstrap and ASP.Net Core would say it is obvious that that should work but it was not obvious to me. When I searched for answers I just got very confused; I found stuff implying I had to re-compile stuff just to change the style. Microsoft tends to document stuff using tutorials and this is the type of thing that should be in a tutorial of ASP.Net Core Web Apps generated by VS (and/or the dotnet command).
The article Create a user-defined service says to use a MS-DOS command prompt but MS-DOS is archaic and not supported by current Windows. The article also describes using INSTSRV.EXE that I cannot find in my system and I cannot find instructions from Microsoft describing how to install it.
For the Host ASP.NET Core in a Windows Service article I got as far as IHostBuilder.UseWindowsService is called when building the host but I have no idea if that is describing something already in the project (it is not) or if I am supposed to add it. If I am supposed to add it then I have no idea where.
Yes I know how frustrating it is when things like that happen. In that thread the member requesting assistance thought that nothing in your response was relevant. I agree that instead of posting the link (with very little other explanation) from your answer as a answer, a comment could have been posted to your answer.
I think that Microsoft Q&A has become less popular. Microsoft is using software designed to work like StackOverflow but Microsoft is not managing Q&A in the same way. And in my opinion at least the StackOverflow design is not what Microsoft should be using; StackOverflow discourages discussion. Also, StackOverflow is designed to create a reference source independent of the documentation and Microsoft should use a system that supports and encourages fixing the documentation.