Added default theme (#960)

* feat: Added default theme

* Revert "feat: Added default theme"

This reverts commit a11a99f92a.

* feat: Add basic theme feature

* feat: Add theme settings

* fix: Fix default theme value

* fix: Fix CR and doc-string

* Comment out darkMode on exampleSite/hugo.yaml

Signed-off-by: hossainemruz <hossainemruz@gmail.com>

---------

Signed-off-by: hossainemruz <hossainemruz@gmail.com>
Co-authored-by: Emruz Hossain <hossainemruz@gmail.com>

layouts/partials/helpers/get-esbuild-options.html

@@ -69,15 +69,26 @@ params:
         #  id: foo
         #  name: bar
     
-    # The `darkMode` feature
-    darkmode:
+    # [Deprecated] The `darkMode` feature
+    # This is a deprecated setting, but has not been removed to maintain backward compatibility
+    # If `theme` is set, the `darkMode` setting will be discarded.
+    darkMode:
       enable: true
+  
+    # The `theme` feature
+    theme:
+      enable: true
+      services:
+        light: true # enable light theme. default "true"
+        dark: true # enable dark theme. default "true"
+        default: system # can be either light, dark or system. default "system"
 
 This helper will convert the above config into the following env vars:
 
   * `FEATURE_ANALYTICS=1`
   * `FEATURE_ANALYTICS_GOOGLE=1`
   * `FEATURE_DARKMODE=1`
+  * `FEATURE_THEME=1`
 
 In JS, you can use it like this: