Hello F.,
Now, with beta-style, we can modify LineColor and group (or opt) style:
<style>
root {
LineColor black
Shadowing 0
}
sequenceDiagram {
participant {
BackGroundColor white
LineThickness 1
}
group {
LineThickness 1
LineColor maroon
LineStyle 3
padding 50
margin 50
}
groupHeader {
FontStyle plain
BackGroundColor pink
LineColor maroon
LineStyle 3
padding 10
margin 10
}
}
</style>
Here is the result:

[Click to see on the PlantUML online server]
And to compare, here is the same without any style:

[Click to see...]
If that can help,
Regards,
Th.