content: Lighten dark-theme variant of textStyleError{,Code}

Prompted by Greg's feedback that the dark-theme color was pretty
dark:
  https://chat.zulip.org/#narrow/stream/48-mobile/topic/dark.20theme/near/1936853

Author: chrisbobbe

lib/widgets/content.dart

@@ -82,10 +82,10 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
         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)
+      textStyleError: const TextStyle(fontSize: kBaseFontSize, color: Colors.red)
         .merge(weightVariableTextStyle(context, wght: 700)),
       textStyleErrorCode: kMonospaceTextStyle
-        .merge(TextStyle(fontSize: kBaseFontSize, color: Colors.red.shade900)),
+        .merge(const TextStyle(fontSize: kBaseFontSize, color: Colors.red)),
       textStyleInlineCode: kMonospaceTextStyle.merge(TextStyle(
         backgroundColor: const HSLColor.fromAHSL(0.08, 0, 0, 1).toColor())),
       textStyleInlineMath: kMonospaceTextStyle.merge(TextStyle(