Adding this to your App.css
[sourcecode language=”css”]
.ms-siteicon-img {
display: none;
}
.ms-siteicon-a {
display: block;
position: relative;
background-image: url(‘https://placeholdit.imgix.net/~text?txtsize=33&txt=180×64&w=180&h=64’);
background-repeat: no-repeat;
background-position: center;
width: 180px;
height: 64px;
}
[/sourcecode]
will give you this