You, obviously, can't change the existing CSS file.
You need to load another CSS file and write a new rule-set to replace the rules in the existing one.
The trickiest part of this will be making sure that the selector you use is specific enough so that it overwrites the earlier rules.
You can file the exact selector used to apply the rules you want to change using the Inspector built into the developer tools in your browser. Inspect the element and look at the styles.
You can copy/paste the selector directly. That way your selector will be equally specific and, because your stylesheet is loaded after the Bootstrap one, will replace any rules that set the same property.