Update themes-compiling.asciidoc.

Adding tip about non-compiling theme, re-phrased other on-the-fly compilation
instructions.

Change-Id: I196235823ba95723e3a7436c075cb7e9ee9951be

documentation/themes/themes-compiling.asciidoc

 [[themes.compiling.on-the-fly]]
 == Compiling On the Fly
 
-The easiest way to use Sass themes during theme development is to let the Vaadin
-servlet compile them on the run. In this case, the SCSS source files are placed
-in the theme folder. Compilation is done each time the [filename]#styles.css# is
-requested from the server.
+The easiest way to develop Sass themes is to compile them at runtime, when the page is loaded.
+The Vaadin servlet does this automatically if a compiled theme CSS file is not found from the theme folder.
+You need to have the SCSS source files placed in the theme folder.
+The theme is compiled when the [filename]#styles.css# is first requested from the server.
+If you edit the Sass theme, it is recompiled the next time you reload the page.
 
 The on-the-fly compilation takes a bit time, so it is only available when the
 Vaadin servlet is in the development mode, as described in
 all its dependencies to be in the class path of the servlet. At least for
 production, you must compile the theme to CSS, as described next.
 
+NOTE: If the on-the-fly compilation does not seem to work, ensure that your build script has not generated a pre-compiled CSS file.
+If [filename]#styles.css# file is found the servlet, the compilation is skipped.
+Delete such an existing [filename]#styles.css# file and disable theme compilation temporarily.
 
 [[themes.compiling.eclipse]]
 == Compiling in Eclipse
     <property name="src-location" value="src" />
 
     ... other project build definitions ...
-            
+
     <!-- Name of the theme -->
     <property name="theme" value="book-examples"/>
-        
+
     <!-- Compilation result directory -->
     <property name="result" value="build/result"/>
   </target>
     <path id="compile.classpath">
         <!-- Source code to be compiled -->
         <pathelement path="${src-location}" />
-            
+
         <!-- Vaadin libraries and dependencies -->
         <fileset dir="${result}/lib">
             <include name="*.jar"/>
           depends="init, resolve">
     <delete dir="${result}/VAADIN/themes/${theme}"/>
     <mkdir dir="${result}/VAADIN/themes/${theme}"/>
-    	
+
     <java classname="com.vaadin.sass.SassCompiler"
           fork="true">
       <classpath>
   </target>
 </project>
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