curiosityLynx, 1 year ago

For CSS, this is pretty easy:
Leave the original source alone, and make your modifications (and only the modifications) in a separate file that is loaded after the original source.

Example
Say the original source style has something like this:

a:hover {
    color:green;
    text-decoration:underline;
}

and you want to remove that underline.
Just put the following in your modifications file:

a:hover {
    text-decoration:none;
}

Links will continue to become green when you hover over them, but they will no longer get underlined when you hover over them.

If you can't guarantee that your modifications will get loaded after the original, add !important like this:

a:hover {
    text-decoration:none !important;
}