How to add a standard css class without overriding it?

Question

css

I'm just starting to understand layout, so the following question arose.
Given: bootstrap
Task: to glue the footer to the bottom of the page.
Solution: I want to make the property margin-bottom of the body style: *footer height*px;
By default in bootstrap, this property is zero. I understand that I can change it to what I need, but it seems to me that editing the standard framework classes is bad? (correct if I'm wrong).
In this case, you need to write your own class and copy what suits me in the standard implementation and add your own? If so, how can I make sure that my class does not overlap the standard one, but complements it?
If there are other ways, I'd love to know!
Thank you.

Answer 1

Here's an example: jsfiddle.net/IonDen/x6psd6bh
When you declare a new class with the same name, you don't overwrite the previous one, you only add to it, or override some properties in it. This is the strategy that all bootstrap add-ons and themes work on, and you should do the same.

Answer 2

just redefine the rules to what you need.