Doc: selecting and customizing themes

Change-Id: I4fa32f65a6e2051d8e403f2c48d2d6e5385e336d
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

doc/src/howto/creator-cli.qdoc

@@ -136,6 +136,11 @@
             \li  Core plugin: display keyboard shortcuts as popups when you
                  press them. Mostly useful when presenting \QC to someone else.
 
+        \row
+            \li  -theme <default | dark>
+            \li  Core plugin: apply a dark color theme to \QC, without using
+                 stylesheets.
+
         \row
             \li  -debug <pid>
             \li  Debugger plugin: attach to the process with the given process ID.
@@ -201,4 +206,7 @@
     the \l{Specifying Text Editor Settings}{text editor color scheme} and the
     styling of the integrated \QD.
 
+    You can also switch to a dark theme to customize the appearance of widgets,
+    colors, and icons without using stylesheets.
+
     */

doc/src/overview/creator-configuring.qdoc

@@ -98,7 +98,15 @@
 
     \section1 Changing Color Schemes
 
-    You can use \QC with your favorite color scheme that defines how code
+    Themes enable you to customize the appearance of the \QC UI: widgets,
+    colors, and icons.
+
+    To switch from the default theme to a dark theme, select \gui Tools >
+    \gui Options > \gui Environment, and then select \gui {Dark (built-in)} in
+    the \gui Theme field.
+
+    You can use the \QC text and code editors with your favorite color scheme
+    that defines how code
     elements are highlighted and which background color is used. You can select
     one of the predefined color schemes or create custom ones.
     The color schemes apply to highlighting C++ files, QML files, and generic