Compose-box border is subtle in dark mode

[gnprice]

We got this feedback on the current design of the compose box, in dark mode:

I'm using the latest version and I notice that the box design is not highlighted compared to the text you read and it's difficult to understand where you have to start typing the text, you need a higher contrast or change the design […]

Indeed the compose box has a similar background to the message list, and the boundary is a bit hard to see.

Implementation

We'll use this updated design from @terpimost:

if we change "border-bar" in dark mode from 50% black to 10% white I think we have a better contrast of the panel: […]
Also in dark mode we could turn on a shadow […]

Details are in Figma. See also screenshots in the chat thread.

[lakshya1goel]

Hello, I want to work on this issue. I have implemented it as well.

Before	After

[vatsalkeshav]

@zulipbot claim

[yashrajpu]

Please give me this assignment, it will only take about two minutes to complete. Thank you!