Add note about empty Theme annotation.

Apply suggestions from code review

flow-maven-plugin/src/test/java/com/vaadin/flow/plugin/maven/TestComponents.java

@@ -82,7 +82,7 @@ public static class TranslatedImports extends Component {
     }
 
     @Route
-    @Theme(value =Lumo.class, variant = Lumo.DARK)
+    @Theme(value = Lumo.class, variant = Lumo.DARK)
     public static class MainView extends Component {
         ButtonComponent buttonComponent;
         IconComponent iconComponent;

flow-server/src/main/java/com/vaadin/flow/theme/Theme.java

@@ -47,6 +47,9 @@
  * <p>
  * By default {@code com.vaadin.flow.theme.lumo.Lumo} theme is used if it's in
  * the classpath. You may disable theming with {@link NoTheme} annotation.
+ * Adding an empty {@link Theme} is equal to not having a {@link Theme}
+ * annotation and will default to {@code com.vaadin.flow.theme.lumo.Lumo} if
+ * available.
  * <p>
  * {@link Theme} annotation should be added to your root navigation level,
  * {@link RouterLayout} or to the top level @{@link Route}.

flow-server/src/test/java/com/vaadin/flow/server/frontend/NodeTestComponents.java

@@ -109,7 +109,7 @@ public static class TranslatedImports extends Component {
     }
 
     @JsModule("./common-js-file.js")
-    @Theme(value =LumoTest.class, variant = LumoTest.DARK)
+    @Theme(value = LumoTest.class, variant = LumoTest.DARK)
     @Route
     public static class MainLayout implements RouterLayout {
         @Override

flow-server/src/test/java/com/vaadin/flow/server/frontend/scanner/FullDependenciesScannerTest.java

@@ -73,7 +73,7 @@ public String getThemeUrl() {
 
     }
 
-    @Theme(value =FakeLumoTheme.class)
+    @Theme(value = FakeLumoTheme.class)
     public static class ThemedComponent extends Component {
 
     }

flow-server/src/test/java/com/vaadin/flow/server/frontend/scanner/ScannerTestComponents.java

@@ -108,7 +108,7 @@ static abstract class GeneratedComponent extends Component {
     }
 
 
-    @Theme(value =Theme1.class, variant = Theme0.DARK)
+    @Theme(value = Theme1.class, variant = Theme0.DARK)
     @JsModule("./router-layout-1.js")
     public class RouterLayout1 implements RouterLayout {
         @Override
@@ -117,7 +117,7 @@ public Element getElement() {
         }
     }
 
-    @Theme(value =Theme1.class, variant = Theme0.DARK)
+    @Theme(value = Theme1.class, variant = Theme0.DARK)
     @JsModule("./router-layout-2.js")
     public class RouterLayout2 extends RouterLayout1 {
     }
@@ -127,7 +127,7 @@ public static abstract class View0 extends Component {
     }
 
     @Route(value = "")
-    @Theme(value =Theme4.class)
+    @Theme(value = Theme4.class)
     public static class RootViewWithTheme extends Component {
     }
 
@@ -151,7 +151,7 @@ public static class RootView2WithLayoutTheme {
     }
 
     @Route(value = "", layout = RouterLayout2.class)
-    @Theme(value =Theme2.class, variant = Theme2.FOO)
+    @Theme(value = Theme2.class, variant = Theme2.FOO)
     @JsModule("./view-2.js")
     public static class RootViewWithMultipleTheme extends Component {
 
@@ -233,7 +233,7 @@ public void configureInstance(WebComponent<RootViewWithTheme> webComponent, Root
         }
     }
 
-    @Theme(value =Theme2.class)
+    @Theme(value = Theme2.class)
     public static class ThemeExporter extends WebComponentExporter<RootViewWithTheme> {
         public ThemeExporter() {
             super("root-view");

flow-server/src/test/java/com/vaadin/flow/server/frontend/scanner/samples/RouteLayoutComponent.java

@@ -22,7 +22,7 @@
 import com.vaadin.flow.theme.Theme;
 
 @JsModule("foo.js")
-@Theme(value =CustomTheme.class)
+@Theme(value = CustomTheme.class)
 public class RouteLayoutComponent implements RouterLayout {
     @Override
     public Element getElement() {