adding mouse over to link buttons

Dear All,

I added the link buttons from WordPress (black with white writing to my page) I tried to add a mouse over effect to it so I wrote to the additional CSS:

a:hover, a:active {
  background-color: white;
  color:black;
}

But it does not work. Please help me.

Thank you and BR

Angelika

The page I need help with: [log in to see the link]