Image Link in Text Widget: remove the black background effect

Hello, this is my site:

On the right, you can see image links to social networks. They are an html code inside a text widget. The problem is that awful black space/tab that you can see when the mouse goes over them.

How can I edit my style.css file in order to take it off from this widget only without taking off the effect in the other widgets (categories, archives, tags, etc.)?

I remember I followed some instruction in this forum but I couldn’t solve the problem completely because the black background disappeared, but still there was a tiny red underline under the images.

I’m not an html/css expert, so please try to be very clear. Thank you.