You can do it individually:
@startuml
box "Box 1"
participant "Participant 1" as p1
participant "Participant 2" as p2
end box
box "Box 2"
participant "Participant 3" as p3
end box
p1->p2: Message1
alt #transparent Condition 1
p2->p3: Message 2
else Condition 2
p2->p2: Message 3
end
@enduml
or use skinparam SequenceGroupBodyBackgroundColor transparent and skinparam SequenceReferenceBackgroundColor transparent to set it globally.