You should look up javafx. You can attach a css file to a GUI. it is fairly easy to use. I can give you some example code if you want.
These are some good tutorials on how to use it: https://www.youtube.com/watch?v=FLkOX4Eez6o
I use eclipse to create a css file right click the project -> select new -> other -> CSS file
import javafx.application.*;
import java.text.*;
import java.util.*;
import javafx.event.*;
import javafx.scene.*;
import javafx.scene.control.*;
import javafx.scene.layout.*;
import javafx.scene.input.*;
import javafx.stage.*;
import javafx.geometry.*;
public class Display extends Application{
public static void main(String[] args){
launch(args);
}
@Override
public void start(Stage stage) throws Exception{
HBox root = new HBox();
Button button = new Button("button");
Label label = new Label("Label");
root.getChildren().addAll(button, label);
Scene scene = new Scene(root, 700,300);
scene.getStylesheets().add("Style.css");
stage.setScene(scene);
stage.setTitle("Title");
stage.show();
}
}
<———————————CSS File——————————>
.root{
-fx-background-color: linear-gradient(#383838, #838383);
-fx-font-size: 11pt;
}
.label{
-fx-text-fill: #e8e8e8;
}
.button{
-fx-background-color: linear-gradient(#dc9556, #ab4642);
-fx-background-radius: 100;
}
.text-area{
-fx-background-color: white;
-fx-background-radius: 0;
}