怎样用纯html和css更改默认的上传文件按钮样式

<div class="fileUpload btn btn-primary">
    <span>Upload</span>
    <input type="file" class="upload" />
</div>

.fileUpload {
    position: relative;
    overflow: hidden;
    margin: 10px;
}
 
.fileUpload input.upload {  position: absolute;
    top: 0;
    right: 0;
    margin: 0;
    padding: 0;
    font-size: 20px;
    cursor: pointer;
    opacity: 0;
    filter: alpha(opacity=0);
}

document.getElementById("uploadBtn").onchange = function () {
    document.getElementById("uploadFile").value = this.value;
};

<input id="uploadFile" placeholder="Choose File" disabled="disabled" />
<div class="fileUpload btn btn-primary">
    <span>Upload</span>
    <input id="uploadBtn" type="file" class="upload" />
</div>