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The CSS stylesheets can be found in the src/distresources/style/ directory of the main project. Most general-purpose elements, such as buttons, text fields and menus have their styles defined in Base.css. Styles specific to custom UI components are generally placed in separate CSS files, given the same name as the FXML file they relate to (for example, styles specific to Titlebar.fxml are defined in Titlebar.css).
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