It's possible to disable the shadow by element, even tough it's not documented, but some elements won't have their shadow disabled.
For example:
skinparam ActorShadowing false /' This work '/
skinparam UsecaseShadowing false /' This doesn't work '/
PlantUML version 1.2018.00 (Thu Jan 04 20:42:56 CET 2018)
(GPL source distribution)
Java Runtime: OpenJDK Runtime Environment
JVM: OpenJDK 64-Bit Server VM
Java Version: 9-Debian+0-9b181-4bpo91
Operating System: Linux
OS Version: 4.9.0-5-amd64
Default Encoding: UTF-8
Language: en
Country: US
Machine: Debian
PLANTUML_LIMIT_SIZE: 4096
Processors: 4
Max Memory: 826,277,888
Total Memory: 52,428,800
Free Memory: 45,548,024
Used Memory: 6,880,776
Thread Active Count: 1
The environment variable GRAPHVIZ_DOT has not been set
Dot executable is /usr/bin/dot
Dot version: dot - graphviz version 2.38.0 (20140413.2041)
Installation seems OK. File generation OK