In order to remove the white at the top, the card container should have no background, image container and the content container100% of width. Then you can add background color to your content container.
<div class="card-box"> <div class="image-container"> your image goes here </div> <div class="content-container"> content goes here </div> </div>For the Wave background, you should create a div or main tag then use your stylesheet to give it the wave background and make sure your body has the same background you've used.
<main> <div class="card-box"></div> </main>or
<div class="main"> <div class="card-box"></div> </div>