Our team is attempting to "modernize" an old ASP.NET Web Forms application. One aspect we want to improve is its visual style as it looks very dated by today's standards. Additionally, its existing CSS stylesheet is an abomination and it would be easier to throw it out and start from scratch. Sadly, none of us are CSS experts so we are considering going with a CSS framework such as Bootstrap for simplicity.
My concern is directly using the Bootstrap CSS classes within the application's pages. Keep in mind this is an enterprise-level application and has many, many pages so Bootstrap class usage would be prolific. Is this a bad idea? Should we, instead, create an abstraction over top of Bootstrap? Is this even possible with CSS and/or worth the effort? My feeling is that, sure, even if we did create an abstraction to Bootstrap we probably could not simply swap it out with some other framework later. So what would be the point?