How do I make one CSS stylesheet override another?

I am still pretty new at web design in general, so please bear with me here...

I am designing a kind of Reddit alternative site (kind of like Voat) for a hobby project of mine. And like Reddit I plan to allow my moderators to customize the CSS on their own communities.

I am using a mix of Bootstrap and my own CSS on the default design, and what I find really annoying about Bootstrap is that it always likes to come out on top of all other CSS styling.

My question is how does Reddit usually handle custom styles (just as a bit of inspiration for when I code my own), and how do I explicitly make one stylesheet override all others without the user needing to put !important on every single line? Is it even a good idea to be using css libraries like bootstrap to style a webpage when I will be allowing custom stylesheets?

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