msglist: Follow web's change making dark mode's --color-text-default opaque

In zulip/zulip@8741c9e43, web changed --color-text-default from
`hsl(0deg 0% 100% / 75%)` to `hsl(0deg 0% 85%)`, to fix some bugs:
- literal Unicode emoji were not fully opaque
- strikethroughs appeared uneven; opacity was compounded where
  letters and strikethroughs overlapped.

The change also made message text a little bit higher contrast, and
that seems like an improvement. Discussion:
  https://chat.zulip.org/#narrow/stream/101-design/topic/Message.20text.20color.20in.20dark.20theme/near/1936707

To carry over this change, this commit updates
ContentTheme.textStylePlainParagraph, and also all our other colors
that were following web where the web counterpart was based on
--color-text-default. I believe that turns out to just be
MessageListTheme.messageTimestamp, added in beee77e; see that
commit message.

Before this, emoji in messages were not fully opaque in dark mode
(#953), as Greg reported:
  https://chat.zulip.org/#narrow/stream/48-mobile/topic/dark.20theme/near/1924392
and that's fixed now.

Fixes: #953

Author: chrisbobbe

lib/widgets/content.dart

@@ -68,7 +68,7 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
       colorMessageMediaContainerBackground: const HSLColor.fromAHSL(0.03, 0, 0, 1).toColor(),
       colorThematicBreak: const HSLColor.fromAHSL(1, 0, 0, .87).toColor().withValues(alpha: 0.2),
       textStylePlainParagraph: _plainParagraphCommon(context).copyWith(
-        color: const HSLColor.fromAHSL(0.75, 0, 0, 1).toColor(),
+        color: const HSLColor.fromAHSL(1, 0, 0, 0.85).toColor(),
         debugLabel: 'ContentTheme.textStylePlainParagraph'),
       codeBlockTextStyles: CodeBlockTextStyles.dark(context),
       textStyleError: TextStyle(fontSize: kBaseFontSize, color: Colors.red.shade900)

lib/widgets/message_list.dart

@@ -58,7 +58,7 @@ class MessageListTheme extends ThemeExtension<MessageListTheme> {
       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(0.6, 0, 0, 1).toColor(),
+      messageTimestamp: const HSLColor.fromAHSL(0.8, 0, 0, 0.85).toColor(),
       recipientHeaderText: const HSLColor.fromAHSL(0.8, 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(),