Diving into the source code, you may find how to change the color of Page Background.
<div class="visualBackgroundContainer" style="opacity: 1; background-color: rgb(255, 255, 255);"></div>
Based on my research, the new Power BI JavaScript API will allow you to write code that can securely pass through the iframe boundary, so that your application can programmatically perform an action in a report and listen for events from actions that users make from within the reports themselves. However, I do not find a way to change the background color for the time being. You may submit an idea or leave a comment at GitHub.