Hello B., and all,
It is due to the fact this theme must be used with a dark background, as:
!$BGCOLOR = gray
!theme spacelab
a->b: test
If you want to use on light background, you can use:
skinparam arrowFontColor $PRIMARY_DARK
Then here is the expected result:
!theme spacelab
skinparam arrowFontColor $PRIMARY_DARK
a->b: test
See similar issue here... :
If you want: you can ask directly the author of those themes.
If that can help,
Regards,
Th.