.btn {
background-color: red !important
}

When I doubt..

@Trey50Daniel Can't you just write a rule with stronger specificity then?

@Trey50Daniel I understand that, but to me it seems like writing a more specific CSS rule in Bootstrap is still just possible, so for example instead of

#thing {
color: blue !important
}

you'd just write

#thing #isBlue {
color: blue;
}

You could just add your custom style sheet after you call the bootstrap one. Then any of your styles will over write the bootstrap ones

Or better yet, get the bootstrap less files, edit the variables, recompile...

@Greggergalactic CSS Tricks explains it better than I can in 1000 characters. https://css-tricks.com/when-using-i...

TL;DR important causes downward spirals and should be avoided when possible which it very much is in Bootstrap

!important is my version of a war flashback. The horror!