HTML code duplication problem

I have a dozen pages with diferent contents and every single one have html with css classes and calls for c# methods that then implement more code with simple strings.

 

The problem is I find myself fixing html classes and CSS almost at the end of a new website and i start to thing that i did it all wrong because i have to fix it in all pages. 

 

What architecture should i use and where can i document myself about it please? 

 

I am using ASP.NET WebForms. Should i switch to MVC? razor? what? (not sure what razor is)

 

Thank you