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

@@ -55,7 +55,7 @@ class MessageListTheme extends ThemeExtension<MessageListTheme> {
 
   MessageListTheme.dark() :
     this._(
-      dateSeparator: Colors.white,
+      dateSeparator: Colors.white.withValues(alpha: 0.2),
       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(),

lib/widgets/theme.dart

@@ -186,7 +186,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       contextMenuItemBg: const Color(0xff7977fe),
       contextMenuItemText: const Color(0xff9398fd),
       editorButtonPressedBg: Colors.white.withValues(alpha: 0.06),
-      foreground: const Color(0xffffffff),
+      foreground: const Color(0xff0f0f0f),
       icon: const Color(0xff7977fe),
       iconSelected: Colors.white.withValues(alpha: 0.8),
       labelCounterUnread: const Color(0xffffffff).withValues(alpha: 0.73),