.local/share/jupyter/nbextensions/exercise/main.css

.highlight-mask
{
    background: transparent url('../images/theme/transBlack75.png') repeat 0 0;
    display: none;
    position: absolute;
}
.highlight-drag
{
    background-color: transparent;
    border: dashed #ff3333 3px;
    position: absolute;
    display: none;
}