mermaid/src/themes/class.scss

g.classGroup text {
  fill: $nodeBorder;
  stroke: none;
  font-family: 'trebuchet ms', verdana, arial;
  font-size: 10px;

  .title {
    font-weight: bolder;
  }
}

g.classGroup rect {
  fill: $nodeBkg;
  stroke: $nodeBorder;
}

g.classGroup line {
  stroke: $nodeBorder;
  stroke-width: 1;
}

.classLabel .box {
  stroke: none;
  stroke-width: 0;
  fill: $nodeBkg;
  opacity: 0.5;
}

.classLabel .label {
  fill: $nodeBorder;
  font-size: 10px;
}

.relation {
  stroke: $nodeBorder;
  stroke-width: 1;
  fill: none;
}

@mixin composition {
  fill: $nodeBorder;
  stroke: $nodeBorder;
  stroke-width: 1;
}

#compositionStart {
  @include composition;
}

#compositionEnd {
  @include composition;
}

@mixin aggregation {
  fill: $nodeBkg;
  stroke: $nodeBorder;
  stroke-width: 1;
}

#aggregationStart {
  @include aggregation;
}

#aggregationEnd {
  @include aggregation;
}

#dependencyStart {
  @include composition;
}

#dependencyEnd {
  @include composition;
}

#extensionStart {
  @include composition;
}

#extensionEnd {
  @include composition;
}