fix: set a default color for text in modal dialogs

Some atlaskit components, such as field-text, inherit text color.
This is a problem with components that are white as they will
inherit $defaultColor, which is a light gray. So instead, for
the atlaskit modal, set a color for all the form content so it
can be inherited instead.

css/_variables.scss

  * Modals
  */
 $modalButtonFontSize: 14px;
+$modalTextColor: #333;
 
 /**
  * Notifications

css/modals/_dialog.scss

     }
 }
 
+.modal-dialog-form {
+    color: $modalTextColor;
+}
 .modal-dialog-footer {
     font-size: $modalButtonFontSize;
 }

css/modals/speaker_stats/_speaker_stats.scss

 .speaker-stats {
     list-style: none;
     padding: 0;
-    color: $auiDialogColor;
     width: 100%;
     font-weight: 500;
 

react/features/base/dialog/components/Dialog.web.js

                 width = { this.props.width || 'medium' }>
                 <div>
                     <form
+                        className = 'modal-dialog-form'
                         id = 'modal-dialog-form'
                         onSubmit = { this._onSubmit }>
                         { this.props.children }