Подробнее описана глава "Тема в десктоп-приложениях"

doc/content/manual/ru/source/section_gui/themes/desktop/example.txt

@@ -1,2 +1,2 @@
 cuba.desktop.resourceLocations = com/haulmont/cuba/desktop/res \
-                                 com/haulmont/ext/desktop/res
+                                 com/sample/sales/desktop/res

doc/content/manual/ru/source/section_gui/themes/desktop/example4.txt

@@ -0,0 +1,14 @@
+public class UnderlinedLabelDecorator implements ComponentDecorator {
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public void decorate(Object component, Set<String> state) {
+        DesktopLabel item = (DesktopLabel) component;
+        JLabel jlabel = item.getComponent();
+
+        Font originalFont = jlabel.getFont();
+        Map attributes = originalFont.getAttributes();
+        attributes.put(TextAttribute.UNDERLINE, TextAttribute.UNDERLINE_ON);
+        jlabel.setFont(originalFont.deriveFont(attributes));
+    }
+}

doc/content/manual/ru/source/section_gui/themes/desktop/example5.txt

@@ -0,0 +1,5 @@
+<theme xmlns="http://schemas.haulmont.com/cuba/4.0/desktop-theme.xsd">
+    <style name="label-underlined" component="com.haulmont.cuba.desktop.gui.components.DesktopLabel">
+        <custom class="com.sample.sales.desktop.gui.decorators.UnderlinedLabelDecorator"/>
+    </style>
+</theme>

doc/content/manual/ru/source/section_gui/themes/desktop/example6.txt

@@ -0,0 +1 @@
+<label id="label1" stylename="label-underlined" value="Label"/>