Forced colors mode WebDriver emulation command (#905)

* forced colors mode emulation

* add parame restriction for theme

* add comma

* make theme data name more specific

* remove first two steps of algo

* add logic for setting fcm emulation in browsingContext.setViewport command

* pre-commit error

* address feedback about method params

* address feedback

* update cddl

* update ordering

* Clean up merge in index.bs

* remove null

* Revert "remove null"

This reverts commit 886e84f.

* address latest feedback

* context->contexts

* update to only convert null once

* fix ordering

Author: alisonmaher

index.bs

@@ -4892,6 +4892,12 @@ TODO: Move it as a hook in the html spec instead.
    1. If [=geolocation overrides map=] [=map/contains=] |user context|,
       [=set emulated position data=] with |navigable| and [=geolocation overrides map=][|user context|].
 
+   1. If [=forced colors mode theme overrides map=] [=map/contains=] |user context|:
+
+      1. Let |theme| be [=forced colors mode theme overrides map=][|user context|].
+
+      1. [=Set emulated forced colors theme data=] with |navigable| and |theme|.
+
    1. If [=screen orientation overrides map=] [=map/contains=] |user context|,
       [=set emulated screen orientation=] with |navigable| and
       [=screen orientation overrides map=][|user context|].
@@ -5813,13 +5819,17 @@ relating to emulation of browser APIs.
 
 <pre class="cddl" data-cddl-module="remote-cddl">
 EmulationCommand = (
+  emulation.SetForcedColorsModeThemeOverride //
   emulation.SetGeolocationOverride //
   emulation.SetLocaleOverride //
   emulation.SetScreenOrientationOverride //
   emulation.SetTimezoneOverride
 )
 </pre>
 
+A [=remote end=] has a <dfn>forced colors mode theme overrides map</dfn> which is a weak map
+between [=user contexts=] and string or null.
+
 A <dfn>geolocation override</dfn> is a [=struct=] with:
 * [=struct/item=] named <dfn attribute for="geolocation override">latitude</dfn> which is a float;
 * [=struct/item=] named <dfn attribute for="geolocation override">longitude</dfn> which is a float;
@@ -5840,6 +5850,81 @@ A [=remote end=] has a <dfn>screen orientation overrides map</dfn> which is a we
 map between [=user contexts=] and [=screen orientation override=].
 
 ### Commands ### {#module-emulation-commands}
+#### The emulation.setForcedColorsModeThemeOverride Command ####  {#command-emulation-setForcedColorsModeThemeOverride}
+
+The <dfn export for=commands>emulation.setForcedColorsModeThemeOverride</dfn> command modifies
+[=forced colors mode=] theming characteristics on the given top-level traversables or user contexts.
+
+<dl>
+   <dt>Command Type</dt>
+   <dd>
+    <pre class="cddl" data-cddl-module="remote-cddl">
+      emulation.SetForcedColorsModeThemeOverride = (
+        method: "emulation.setForcedColorsModeThemeOverride",
+        params: emulation.SetForcedColorsModeThemeOverrideParameters
+      )
+
+      emulation.SetForcedColorsModeThemeOverrideParameters = {
+        theme: emulation.ForcedColorsModeTheme / null,
+        ? contexts: [+browsingContext.BrowsingContext],
+        ? userContexts: [+browser.UserContext],
+      }
+
+      emulation.ForcedColorsModeTheme = "light" / "dark"
+    </pre>
+   </dd>
+   <dt>Result Type</dt>
+   <dd>
+    <code>
+     EmptyResult
+    </code>
+   </dd>
+</dl>
+
+Note: Check out the {{ForcedColorsModeAutomationTheme}} for the corresponding enum mapping in the CSS specification.
+
+<div algorithm="remote end steps for emulation.setForcedColorsModeThemeOverride">
+
+The [=remote end steps=] with |command parameters| are:
+
+1. If |command parameters| [=map/contains=] "<code>userContexts</code>"
+   and |command parameters| [=map/contains=] "<code>contexts</code>",
+   return [=error=] with [=error code=] [=invalid argument=].
+
+1. If |command parameters| doesn't [=map/contain=] "<code>userContexts</code>"
+   and |command parameters| doesn't [=map/contain=] "<code>contexts</code>",
+   return [=error=] with [=error code=] [=invalid argument=].
+
+1. Let |theme| be |command parameters|["<code>theme</code>"].
+
+1. If |theme| is null, set |theme| to "<code>none</code>".
+
+1. Let |navigables| be a [=/set=].
+
+1. If the <code>contexts</code> field of |command parameters| is present:
+
+   1. Let |navigables| be the result of [=trying=] to [=get valid top-level traversables by ids=] with |command parameters|["<code>contexts</code>"].
+
+1. Otherwise:
+
+   1. Let |user contexts| be the result of [=trying=] to [=get valid user contexts=] with |command parameters|["<code>userContexts</code>"].
+
+   1. For each |user context| of |user contexts|:
+
+      1. [=map/Set=] [=forced colors mode theme overrides map=][|user context|] to |theme|.
+
+      1. [=list/For each=] |top-level traversable| of the list of all [=/top-level traversables=]
+         whose [=associated user context=] is |user context|:
+
+         1. [=list/Append=] |top-level traversable| to |navigables|.
+
+1. For each |navigable| of |navigables|:
+
+   1. [=Set emulated forced colors theme data=] with |navigable| and |theme|.
+
+1. Return [=success=] with data null.
+
+</div>
 
 #### The emulation.setGeolocationOverride Command ####  {#command-emulation-setGeolocationOverride}