UI dark mode: Low contrast for the color red in rustc error messages

[fmease]

This concerns elements with CSS classes token error in the dark UI mode.

Contrast between foreground color #BF1B1B and background color #343434 is 2:1 thus failing WCAG AA (according to WebAIM).

I haven't checked the other colors that may appear in rustc's diagnostics.

Edit: I actually forgot to say explicitly that I personally find it hard to read. The numbers were just meant to back stuff up ^^

[fmease]

Ah, I've just noticed #1087. However, it doesn't seem to address this specific issue (yet).

[shepmaster]

it doesn't seem to address this specific issue

not directly, but it does change the output background to #191b1c

Thoughts?

Chrome (with Advanced Perceptual Contrast Algorithm (APCA) enabled) does not complain about the error text now (while it does for many other things, such as the warning text)

[fmease]

That's a lot more legible 👍 I guess you can mark your PR as fixing this issue.

Still, as alluded to, colors other than red (like yellow/orange, purple, green) might need pop a little bit more when the UI is set to dark, tho I haven't tested that with the new, darker background color.