Added chat window transparency slider to options (#24990)

* Adds a new slider to the misc tab in options that lets the player set chat window transparency

* Tweaked variable names

* Fixed order to match UI

* Renamed set chat window transparency function

* Changed and refactored to opacity instead of transparency

* Remove unnecessary int to float conversions

Slider used to be 0-100 while the CCVar was 0.0-1.0f. This is confusing and was only used for rounding to 2 decimal points.

* Round the value to two decimal points

* Remove rounding for now

* Rename

* Unhardcode chat color by moving to stylesheet

* Fix indent

* Make opacity slider only change opacity

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Co-authored-by: Your Name <you@example.com>
Co-authored-by: ShadowCommander <10494922+ShadowCommander@users.noreply.github.com>

Content.Shared/CCVar/CCVars.cs

@@ -1580,6 +1580,13 @@ namespace Content.Shared.CCVar
         * Accessibility
         */
 
+        /// <summary>
+        /// Chat window opacity slider, controlling the alpha of the chat window background.
+        /// Goes from to 0 (completely transparent) to 1 (completely opaque)
+        /// </summary>
+        public static readonly CVarDef<float> ChatWindowOpacity =
+            CVarDef.Create("accessibility.chat_window_transparency", 0.85f, CVar.CLIENTONLY | CVar.ARCHIVE);
+
         /// <summary>
         /// Toggle for visual effects that may potentially cause motion sickness.
         /// Where reasonable, effects affected by this CVar should use an alternate effect.