Best way to allow users to override plugin styles

Hi there,
I’m writing a slider plugin for a client and want to allow them to override specific styles without replacing the whole stylesheet. I know of a few plugins that work like this (The Events Calendar is one example). The ideal behavior would be: if a stylesheet with a specific name exists, give styles from that file precedence over the plugin’s orifignal files.

I’m assuming I would do something like:

if (custom-styles.css exists) {
load stylesheet;
}

…but, what happens when both stylesheets have a rule for the same element?
Is there some way to force the custom-styles.css file to be more specific?

Thanks in advance for any advice!
-Justin