The interface theme defines the appearance of windows, dialogs, and controls. By default, IntelliJ IDEA uses the Darcula theme, unless you changed it during the first run.
Change the UI theme
In the Settings/Preferences dialog (Ctrl+Alt+S), select Appearance & Behavior | Appearance.
Select the UI theme from the Theme list:
Darcula: Default dark theme
Light: More traditional light theme
High contrast: Theme designed for users with color vision deficiency
It is also possible to create your own UI themes for IntelliJ IDEA and customize the built-in themes. For more information, see IntelliJ Platform SDK Documentation.