Final fix the dark theme

Author: OEvgeny

__tests__/html2/fluentTheme/defaultFeedback.activity.dark.html

@@ -59,7 +59,8 @@
             }),
             colorNeutralBackground1Disabled: '#101010',
             colorNeutralBackground1Hover: '#101010',
-            colorNeutralForeground5: '#424242'
+            colorNeutralForeground5: '#424242',
+            colorNeutralForegroundOnBrand: '#292929'
           };
 
           root.render(

packages/fluent-theme/src/components/theme/Theme.module.css

@@ -21,6 +21,8 @@
   --webchat-colorNeutralForeground4: var(--colorNeutralForeground4, #707070);
   --webchat-colorNeutralForeground5: var(--colorNeutralForeground5, #7e7e7e);
 
+  --webchat-colorNeutralForegroundOnBrand: var(--colorNeutralForegroundOnBrand, #fff);
+
   --webchat-colorNeutralBackgroundDisabled: var(--colorNeutralBackgroundDisabled, #f0f0f0);
   --webchat-colorNeutralBackground1: var(--colorNeutralBackground1, #ffffff);
   --webchat-colorNeutralBackground1Hover: var(--colorNeutralBackground1Hover, #f5f5f5);
@@ -645,7 +647,7 @@
   background-color: var(--webchat-colorBrandForeground1);
   border: var(--webchat-strokeWidthThin) solid var(--webchat-colorBrandForeground1);
   border-radius: var(--webchat-borderRadiusMedium);
-  color: var(--webchat-colorNeutralBackground1);
+  color: var(--webchat-colorNeutralForegroundOnBrand);
   cursor: pointer;
   font-family: var(--webchat-fontFamilyBase);
   font-size: var(--webchat-fontSizeBase300);