need to make login / register button fade when rolled over

Hi, I have used a couple of methods to get the red link in the right hand sidebar on the homepage of the above site to fade to 50% when rilled over but it just won't do it

Can you help pls. I have used two different CSS possibilities which are in the child CSS file but no luck. something is stopping the code from working that is working for other buttons on the page.

Thanks In advance.