Any way to change the white "gutter color" between pages in IN_LINE mode? (Or remove entirely?)

Thanks for using View SDK. There is no way you can remove the gutter that appears between the PDF, although you can change the color by providing the hashcode of your desired color as shown below.

<div id="adobe-dc-view" style="width: 800px;"></div>
<script src="https://documentcloud.adobe.com/view-sdk/main.js"></script>
<script type="text/javascript">
	document.addEventListener("adobe_dc_view_sdk.ready", function(){ 
		var adobeDCView = new AdobeDC.View({
                       clientId: "<YOUR_CLIENT_ID>", 
                       divId: "adobe-dc-view", 
                       backgroundColor: "<HASH_CODE_FOR_COLOR>"
                });
		adobeDCView.previewFile({
			content:{location: {url: "https://documentcloud.adobe.com/view-sdk-demo/PDFs/Bodea Brochure.pdf"}},
			metaData:{fileName: "Bodea Brochure.pdf"}
		}, {embedMode: "IN_LINE"});
	});
</script>