Override css rules of a plugin within my own theme

The style_loader_src filter is used to modify/filter the src (url) of a stylesheet. The function callback accepts two parameters, the first is the original src and the second the handler of the stylesheet. The handler is the first parameter of wp_enqueue_style() used to enqueue the stylesheet.

So, to replace a stylsheet source with another one you should check the handler to know wchich stylesheet it must replaced, in the code you posted the checked handler is plugin-script-handle.