use "variables" in .properties files

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -40,6 +40,9 @@ import com.formdev.flatlaf.util.SystemInfo;
 public abstract class FlatLaf
 	extends BasicLookAndFeel
 {
+	private static final String VARIABLE_PREFIX = "@";
+	private static final String GLOBAL_PREFIX = "*.";
+
 	private BasicLookAndFeel base;
 
 	@Override
@@ -143,14 +146,14 @@ public abstract class FlatLaf
 			}
 
 			// get globals, which override all other defaults that end with same suffix
-			String globalPrefix = "*.";
 			HashMap<String, Object> globals = new HashMap<>();
-			for( Object okey : properties.keySet() ) {
-				String key = (String) okey;
-				if( key.startsWith( globalPrefix ) ) {
-					Object value = parseValue( key, properties.getProperty( key ) );
-					globals.put( key.substring( globalPrefix.length() ), value );
-				}
+			for( Map.Entry<Object, Object> e : properties.entrySet() ) {
+				String key = (String) e.getKey();
+				if( !key.startsWith( GLOBAL_PREFIX ) )
+					continue;
+
+				String value = resolveVariable( properties, (String) e.getValue() );
+				globals.put( key.substring( GLOBAL_PREFIX.length() ), parseValue( key, value ) );
 			}
 
 			// override UI defaults with globals
@@ -167,7 +170,10 @@ public abstract class FlatLaf
 			// add non-global properties to UI defaults
 			for( Map.Entry<Object, Object> e : properties.entrySet() ) {
 				String key = (String) e.getKey();
-				String value = (String) e.getValue();
+				if( key.startsWith( VARIABLE_PREFIX ) || key.startsWith( GLOBAL_PREFIX ) )
+					continue;
+
+				String value = resolveVariable( properties, (String) e.getValue() );
 				defaults.put( key, parseValue( key, value ) );
 			}
 		} catch( IOException ex ) {
@@ -175,6 +181,17 @@ public abstract class FlatLaf
 		}
 	}
 
+	private String resolveVariable( Properties properties, String value ) {
+		if( !value.startsWith( VARIABLE_PREFIX ) )
+			return value;
+
+		String newValue = properties.getProperty( value );
+		if( newValue == null )
+			System.err.println( "variable '" + value + "' not found" );
+
+		return newValue;
+	}
+
 	private Object parseValue( String key, String value ) {
 		value = value.trim();
 

flatlaf-core/src/main/resources/com/formdev/flatlaf/FlatDarkLaf.properties

@@ -14,27 +14,32 @@
 # limitations under the License.
 #
 
+#---- variables ----
+
+@background=3c3f41
+@foreground=bbbbbb
+
 #---- globals ----
 
-*.background=3c3f41
-*.foreground=bbbbbb
-*.textBackground=3c3f41
-*.textForeground=bbbbbb
-*.caretForeground=bbbbbb
-*.inactiveBackground=3C3F41
-*.inactiveForeground=bbbbbb
+*.background=@background
+*.foreground=@foreground
+*.textBackground=@background
+*.textForeground=@foreground
+*.caretForeground=@foreground
+*.inactiveBackground=@background
+*.inactiveForeground=@foreground
 *.selectionBackground=4B6EAF
-*.selectionForeground=bbbbbb
+*.selectionForeground=@foreground
 
 
 #---- system ----
 
-control=3c3f41
-controlText=bbbbbb
-infoText=bbbbbb
-text=bbbbbb
-textText=bbbbbb
-window=3c3f41
+control=@background
+controlText=@foreground
+infoText=@foreground
+text=@foreground
+textText=@foreground
+window=@background
 
 
 #---- Label ----

flatlaf-core/src/main/resources/com/formdev/flatlaf/FlatLightLaf.properties

@@ -14,13 +14,18 @@
 # limitations under the License.
 #
 
+#---- variables ----
+
+@background=f2f2f2
+@foreground=000000
+
 #---- globals ----
 
-*.background=f2f2f2
-*.foreground=000000
+*.background=@background
+*.foreground=@foreground
 *.textBackground=cccccc
-*.textForeground=000000
-*.caretForeground=000000
+*.textForeground=@foreground
+*.caretForeground=@foreground
 *.inactiveBackground=ffffff
 *.inactiveForeground=777777
 *.selectionBackground=4A6EB7
@@ -30,11 +35,11 @@
 #---- system ----
 
 control=e0e0e0
-controlText=000000
-infoText=000000
-text=000000
-textText=000000
-window=f2f2f2
+controlText=@foreground
+infoText=@foreground
+text=@foreground
+textText=@foreground
+window=@background
 
 
 #---- Label ----