mermaid/src/themes/flow.scss

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

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

.node.clickable {
  cursor: pointer;
}

.arrowheadPath {
  fill: $arrowheadColor;
}

.edgePath .path {
  stroke: $lineColor;
  stroke-width: 1.5px;
}

.edgeLabel {
  background-color: $edgeLabelBackground;
}

.cluster rect {
  fill: $secondBkg !important;
  rx: 4 !important;
  stroke: $clusterBorder !important;
  stroke-width: 1px !important;
}

.cluster text {
  fill: $titleColor;
}