Tweak dark theme for slightly higher contrast

[gnprice]

Our colors in dark theme are based on those in web. The dark theme in web is getting adjusted to have slightly higher contrast, and we should follow. Details:

• https://chat.zulip.org/#narrow/stream/137-feedback/topic/design.20feedback/near/1950672
• dark_theme: Adjust message colors for higher contrast. zulip#31806

For mobile, the specific design from @terpimost can be found here:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4514-33396&t=bMNXsr5LwkrHz4dp-1

@terpimost would you suggest using the exact same values for these in mobile as in web? Or would you prefer to update things in the mobile Figma doc?

Related (but independent) PR

• ui: Make some dark-theme adjustments from Greg's feedback #955

[IfeanyiSam]

Is this issue open to work on? I want to work on this issue if it is open

[donnywellson]

@zulipbot claim

[donnywellson]

I would like to be assigned this issue. I have already started by grouping the theme files and makeing the ZulipTheme easier to read and understand

[gnprice]

I've just updated the issue description with the design freshly provided by @terpimost, so it's possible now to work on this issue.

@donnywellson assigning to you; please go ahead. I'm not sure we'll want to merge rearrangements to the theme code; but as long as you keep them in separate commits, following our style for clean and coherent commits, that won't get in the way of reviewing your main changes.

@IfeanyiSam please take a look at our guide for claiming an issue:
https://zulip.readthedocs.io/en/latest/contributing/contributing.html#in-other-zulip-repositories