At the moment this beautiful feature is missing in GANTT diagram.
It can be useful and nice, for example
<style>
ganttDiagram {
BackGroundColor lightCyan
FontName Consolas
}
</style>
(--------)
With last beta http://beta.plantuml.net/plantuml.jar, you can now have
@startuml <style> ganttDiagram { task { FontName Helvetica FontColor red FontSize 18 FontStyle bold BackGroundColor blue LineColor yellow } milestone { FontColor blue FontSize 25 FontStyle italic BackGroundColor yellow LineColor red } } </style> [Task] lasts 20 days [M1] happens on 5 days after [Task]'s end @enduml
Not all of the style properties are includes. For example:
RoundCorder, Margin, Padding do not work when defining a style