We had a similar issue, I would also suggest to update Atlas first, as well as datawidgets. If that doesn't solve it.
This might help.
.mx-icon-filled {
font-family: 'Glyphicons Halflings';
font-style: normal;
font-weight: normal;
speak: none;
display: inline-block;
text-decoration: inherit;
width: 1em;
height: 1em;
line-height: 1em;
vertical-align: -0.125em;
-webkit-font-smoothing: antialiased;
-moz-osx-font-smoothing: grayscale;
}
.mx-icon-controls-forward:before {
content: '\e075';
}
.mx-icon-controls-backward:before{
content: '\e071';
}
.mx-icon-controls-step-backward:before {
content: '\e069';
}
.mx-icon-controls-step-forward:before {
content: '\e077';
}