Agency Page Using Grid and Flexbox

Use pseudo element like ::before to add top triangle:

modified your css below:

#dropdown{
        position: fixed;
        z-index: 100;
        top: 4rem;
        right: 0;
        left: 0;
        background-color: rgba(255, 255, 255, 1);
        margin: 1.5rem;
    }

    #dropdown::before{
        content: "";
        width: 0px;
        height: 0px;
        position: absolute;
        background: transparent;
        border-left: 20px solid transparent;
        border-right: 30px solid #fff;
        border-top: 35px solid transparent;
        right: 0;
        top: -30px;
    }

[Res for Pseudo elements] (https://css-tricks.com/pseudo-element-roundup/) [Res for making triangles like this] (https://css-tricks.com/snippets/css/css-triangle/)