Let custom css override specific class rules, not all classes with the same name, is it possible?

Let custom css override specific class rules, not all classes with the
same name, is it possible?

I´m building a website and using a template. The template has lots of css
files some of these files id like to change and here comes the problem.
When making some new rules in my custom.css file for a specific class, the
new added rules override the matched css rules for that class and not just
for that class but for all the classes with the same name. And here comes
the problem the overriding must only happen at that specific class,
otherwise my entire website will look very strange...
Can this be done?
Marcel