mermaid/src/less/forest/flow.less

.mermaid .label { 
  font-family: 'trebuchet ms', verdana, arial;
color:#333
}

.node rect, .node circle, .node polygon  {
  fill: @mainBkg;
  stroke: @nodeBorder;
  stroke-width: 1px;
}

.edgePath .path {
  stroke: @lineColor;
  stroke-width: 1.5px;
  
}
.cluster rect{
  fill: @secondBkg !important;
  rx:4  !important;
  stroke: @clusterBorder !important;
  stroke-width: 1px !important;
}

.cluster text {
  fill:@titleColor;
}