Change color menu on scrolling

Hi,
I entered this css code to put a transparent navigation bar at the top of the page

.navigation-top {
background-color: transparent;
border: none;
width: 100%;
height: 120px;
top: 0;
font-size: 130%;
bottom: auto;
position: fixed;
left: 0;
right: 0;
z-index: 7;
}

Now I would like the navigation bar to be transparent but when you scroll down the page it changes to color blu.
can someone help me?
Thanks in advance

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