处理具有相同名称的CSS类

I have taken over a project for someone. Thee former developer started using using Bootstrap to create a new menu. The problem is that bootstrap and at least one other CSS file in the project have classes with the same name. Because of this, bootstrap is causing the existing layout to not display properly. What are ways to deal with this type of situation? Bootstrap really just needs to be use with the web page header and nothing else. CSS is not one of my strong skill sets. I have taken over a project for someone. Thee f