Diana,
Sure you will need to adjust the css rules for the jimu-widget-onscreen-icon. Here is an example (green color):
.jimu-widget-onscreen-icon {
position: absolute;
cursor: pointer;
border-radius: 4px;
background-color: rgb(42, 170, 138);
-ms-filter: "Alpha(opacity=20)";
background-color: rgba(42, 170, 138, 0.2);
}
.jimu-widget-onscreen-icon.jimu-state-selected {
background-color: rgb(42, 170, 138);
-ms-filter: "Alpha(opacity=80)";
background-color: rgba(42, 170, 138, 0.8);
}
.jimu-widget-onscreen-icon:hover{
background-color: rgb(42, 170, 138);
-ms-filter: "Alpha(opacity=40)";
background-color: rgba(42, 170, 138, 0.4);
}