Jay,
The color of the preload widget icon is controlled by the css. The way that I handled this is by adding some css rules to the particular theme color style.css for my app.
server\apps\11\themes\FoldableTheme\styles\olive\style.css
/* My Theme Style Adds */
.jimu-preload-widget-icon {
position: absolute;
cursor: pointer;
border-radius: 4px;
background-color: rgb(90, 107, 77);
-ms-filter: "Alpha(opacity=20)";
background-color: rgba(90, 107, 77, 0.2);
}
.jimu-preload-widget-icon.jimu-state-selected {
background-color: rgb(90, 107, 77);
-ms-filter: "Alpha(opacity=80)";
background-color: rgba(90, 107, 77, 0.8);
}
.jimu-preload-widget-icon:hover{
background-color: rgb(90, 107, 77);
-ms-filter: "Alpha(opacity=40)";
background-color: rgba(90, 107, 77, 0.4);
}