…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
(zulip#953), as Greg reported:
  https://chat.zulip.org/#narrow/stream/48-mobile/topic/dark.20theme/near/1924392
and that's fixed now.

Fixes: zulip#953