How to make certain objects hidden when CSS Sticky footer bar floats over

I have a mildly transparent sticky footer that I've taken from CSS bootstrap and I was wondering how to make specific properties within various css selectors have values that make them opacity=0 or similar, (i.e., so that they are hidden) when the transparent sticky footer floats over.

Is there a tried and tested method for this...? Perhaps a javascript method...?

If someone could point me in the right direction or give me a started for 10 that would be much appreciated.