Change colors to match preview of native dark mode

[pbaity]

@fdellwing and a few others of us talked previously (see this comment and following) about improving the colors and contrast of the dark theme. I think this is an excellent idea and we agreed a good starting point would be to emulate the colors of the sample native dark theme shared by the Rocket.Chat team:

There's no rule stipulating we need to copy these exactly, but whoever wants to pick up this issue can start with these colors and see how the community likes your changes.

[pbaity]

@fdellwing Do you still plan on working on this? If not that's fine, just checking in.

[fdellwing]

I already started with a few basics, but I have very little time atm for this at work. Will likely take another 3-4 weeks until I have something to present.

[pbaity]

Great, thanks for working on it, and no rush at all - when you feel like you have enough to share, feel free to open a WIP PR and I can do what I can to test and assist.

[ghost]

I'm in no rush for new colours, but this looks very nice. Looking forward to it.

[chotaire]

I did that a year ago or so. I will see if I can find some time to rebase and merge, then send pull requests. I wish the day had 100 hours.