Hello A.,
From the next version V1.2020.12, see: https://plantuml.com/changes
We can use BackGroundColor transparent, with conjonction of adding a default color on node, as:
@startmindmap
<style>
mindmapDiagram {
BackGroundColor transparent
node {
BackGroundColor #FEFECE
}
}
</style>
* a
** a1
** a2
@endmindmap
But if we use only BackGroundColor, we observe this issue (only on PNG export):
@startmindmap
<style>
mindmapDiagram {
BackGroundColor transparent
}
</style>
* a
** a1
** a2
@endmindmap
If that can help,
Regards,
Th.