Allow me to provide you with an illustration.
Imagine that Bootstrap.css contains the following:
.rules #world {border:0px;}
[specificity 110]
While css.css includes this rule:
div #world{border:20px;}
[specificity 101]
In this scenario, bootstrap.css will consistently take precedence over css.css. My suggestion would be to introduce a container for all your content - assign this container an ID, let's say "god," and then prefix all your styles in CSS with .god - although it's not as straightforward and requires precise calculations.
#god div #world{border:20px;}
It's a complex topic, as someone else mentioned earlier - you need to have a comprehensive understanding of how both Bootstrap and css.css are affecting your page.
You may find this timeless article useful: