theme: Update theme variables for tweak dark theme

This change updates the dark theme variables in
`DesignVariables` and `MessageListTheme` acc. to
their values in Figma.
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4514-33396&t=bMNXsr5LwkrHz4dp-1

CZO Discussion: https://chat.zulip.org/#narrow/stream/137-feedback/topic/design.20feedback/near/1950672

Fixes: #973

lib/widgets/message_list.dart

@@ -56,13 +56,13 @@ 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(),
+      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

@@ -173,7 +173,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       bannerBgIntDanger: const Color(0xff461616),
       bgBotBar: const Color(0xff222222),
       bgContextMenu: const Color(0xff262626),
-      bgCounterUnread: const Color(0xff666699).withValues(alpha: 0.37),
+      bgCounterUnread: const Color(0xff393c49),
       bgMenuButtonActive: Colors.black.withValues(alpha: 0.2),
       bgMenuButtonSelected: Colors.black.withValues(alpha: 0.25),
       bgTopBar: const Color(0xff242424),
@@ -189,12 +189,12 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       foreground: const Color(0xffffffff),
       icon: const Color(0xff7977fe),
       iconSelected: Colors.white.withValues(alpha: 0.8),
-      labelCounterUnread: const Color(0xffffffff).withValues(alpha: 0.7),
+      labelCounterUnread: const Color(0xffffffff).withValues(alpha: 0.73),
       labelEdited: const HSLColor.fromAHSL(0.35, 0, 0, 1).toColor(),
       labelMenuButton: const Color(0xffffffff).withValues(alpha: 0.85),
       mainBackground: const Color(0xff1d1d1d),
       textInput: const Color(0xffffffff).withValues(alpha: 0.9),
-      title: const Color(0xffffffff),
+      title: const Color(0xffffffff).withValues(alpha: 0.9),
       bgSearchInput: const Color(0xff313131),
       textMessage: const Color(0xffffffff).withValues(alpha: 0.8),
       channelColorSwatches: ChannelColorSwatches.dark,
@@ -210,7 +210,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       inboxItemIconMarker: const HSLColor.fromAHSL(0.4, 0, 0, 1).toColor(),
       loginOrDivider: const Color(0xff424242),
       loginOrDividerText: const Color(0xffa8a8a8),
-      modalBarrierColor: const Color(0xff000000).withValues(alpha: 0.5),
+      modalBarrierColor: const Color(0xff000000).withValues(alpha: 0.3),
       // TODO(design-dark) need proper dark-theme color (this is ad hoc)
       mutedUnreadBadge: const HSLColor.fromAHSL(0.5, 0, 0, 0.6).toColor(),
       navigationButtonBg: Colors.white.withValues(alpha: 0.05),