It looks like you have conflicting CSS for the class info, which is where you're getting the weird box.
<div class="info">
<span class="partof">part of <a href="https://ropensci.org"><img src="https://d33wubrfki0l68.cloudfront.net/88338b7637e084df5c43e7a9e6293719dbaf10e2/30903/img/icon_lettering_white.svg" height="24" alt="rOpenSci"></a></span>
<span class="version label label-default version-default" data-toggle="tooltip" data-placement="bottom" title="" data-original-title="Released version">0.2.8</span>
</div>
There's CSS styling for this in both the ropensci.css source file, and in the leafletfix.css source file:
leafletfix.css has
/* Work around CSS properties introduced on img by bootstrap */
.info {
padding: 6px 8px;
font: 14px/16px Arial, Helvetica, sans-serif;
background: white;
background: rgba(255,255,255,0.8);
box-shadow: 0 0 15px rgba(0,0,0,0.2);
border-radius: 5px;
}
which is what's giving you that white box.
The file in the leaflet GitHub repo is below: