Allow users to define a custom CSS file that can override defaults

Allows users to override the 'not_enabled' styling as well as others
used by Phoebus.
Introduces a new property that can be set to the path of the CSS file.

Author: rjwills28

core/ui/src/main/java/org/phoebus/ui/Preferences.java

@@ -95,6 +95,8 @@ public class Preferences
     @Preference public static boolean save_credentials;
     /** documentation_location */
     @Preference public static String documentation_location;
+    /** Custom stylings file */
+    @Preference public static String custom_css_styling;
 
     static
     {

core/ui/src/main/java/org/phoebus/ui/javafx/Styles.java

@@ -7,6 +7,8 @@
  ******************************************************************************/
 package org.phoebus.ui.javafx;
 
+import org.phoebus.ui.Preferences;
+
 import javafx.scene.Node;
 import javafx.scene.Scene;
 
@@ -24,6 +26,10 @@ public static void setSceneStyle(final Scene scene)
     {
         final String css = Styles.class.getResource("csstudio.css").toExternalForm();
         set(scene, css);
+        if (!Preferences.custom_css_styling.isEmpty())
+        {
+            set(scene, Preferences.custom_css_styling);
+        }
     }
 
     /** Apply style sheet to scene (but only once)

core/ui/src/main/resources/phoebus_ui_preferences.properties

@@ -179,3 +179,9 @@ save_credentials=false
 
 # Location of the Phoebus documentation
 documentation_location=
+
+# Custom stylings .css file allows users to override Phoebus and JavaFX
+# default stylings.Set the full path to the .css file
+# For a system file use syntax; 'file:</path/to/custom.css>'
+# For a file served over http use syntax: 'http://<address:port/custom.css>'
+custom_css_styling=