It is all about CSS specificity.
If two properties have the same weight, the latter property is used.
You can override single CSS properties by declaring the property !important
.
Example;
body {
background-color: #000 !important;
}
It is an easy way to overcome CSS selector weight (or specificity) if you notice that Bootstrap styles overrides yours (helpful for a novice just trying out stuff).
But you should read more about specificity instead - http://www.smashingmagazine.com/2007/07/27/css-specificity-things-you-should-know/.