You can choose your preferred theme to apply to Astah from [View] – [Look & Feel] – [Dark Themes] in main menu. Available product and version are as below:
Astah Professional, UML and Viewer version 9.0 and later
Astah System Safety version 8.0 and later
Background of Diagram Editor
Currently, themes do not apply to diagram editor’s background color. If you want to tune it up to fit the dark surface, you can change it from the option below:
Astah Professional and UML : [Tools] – [System Properties] – [Diagram Editor] – [Background color]