Add Dark Mode support for default theme(s)

[MagnusOxlund]

LimeChat now supports Dark Mode as of version 2.46, but it's still using the old default themes, which are arguably more suitable for Light Mode. Here's what the default theme looks like with Dark Mode enabled (with alternating lines):

And here is something a little darker:

The CSS code can be appended to the default CSS theme file inside a media query to automatically match the mode in use: @media (prefers-color-scheme: dark) {}. I don't know how to handle the YAML file, though, so I'm not submitting a pull request. I think it's better if @psychs or a seasoned contributor takes a look at it, if you find it suitable.

I've attached the two theme files. The CSS file includes alternating lines.
Dark Mode (alternating lines).zip