theme: Adjust foreground & dateSeparator colors

Adjusted the colors for `foreground` in
`DesignVariables` and `dateSeparator` in `MessageListTheme`
as asked.
`foreground`: zulip#1213 (comment)
`dateSeparator`: zulip#1213 (comment)

Author: Gaurav-Kushwaha-1225

lib/widgets/message_list.dart

@@ -55,14 +55,14 @@ class MessageListTheme extends ThemeExtension<MessageListTheme> {
 
   MessageListTheme.dark() :
     this._(
-      dateSeparator: Colors.white,
-      dateSeparatorText: const HSLColor.fromAHSL(0.75, 0, 0, 1).toColor(),
-      dmRecipientHeaderBg: const HSLColor.fromAHSL(1, 46, 0.15, 0.2).toColor(),
-      messageTimestamp: const HSLColor.fromAHSL(0.8, 0, 0, 0.85).toColor(),
-      recipientHeaderText: const HSLColor.fromAHSL(0.8, 0, 0, 1).toColor(),
+      dateSeparator: Colors.white.withValues(alpha: 51),
+      dateSeparatorText: const HSLColor.fromAHSL(0.5, 0, 0, 1).toColor(),
+      dmRecipientHeaderBg: const HSLColor.fromAHSL(1, 0, 0, 0.14).toColor(),
+      messageTimestamp: const HSLColor.fromAHSL(0.5, 0, 0, 1).toColor(),
+      recipientHeaderText: const HSLColor.fromAHSL(0.9, 0, 0, 1).toColor(),
       senderBotIcon: const HSLColor.fromAHSL(1, 180, 0.05, 0.5).toColor(),
-      senderName: const HSLColor.fromAHSL(0.85, 0, 0, 1).toColor(),
-      streamMessageBgDefault: const HSLColor.fromAHSL(1, 0, 0, 0.15).toColor(),
+      senderName: const HSLColor.fromAHSL(0.9, 0, 0, 1).toColor(),
+      streamMessageBgDefault: const HSLColor.fromAHSL(1, 0, 0, 0.11).toColor(),
       streamRecipientHeaderChevronRight: Colors.white.withValues(alpha: 0.3),
 
       // 0.75 opacity from here:

lib/widgets/theme.dart

@@ -493,5 +493,5 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
 /// For how this value is cached, see [ChannelColorSwatches.forBaseColor].
 ChannelColorSwatch colorSwatchFor(BuildContext context, Subscription subscription) {
   return DesignVariables.of(context)
-    .channelColorSwatches.forBaseColor(subscription.color);
+      .channelColorSwatches.forBaseColor(subscription.color);
 }