msglist: Implement MessageListTheme, with dark-theme colors - #843

For the background color, the unread-marker color seems like a very
reasonable starting point, since that's what we're using in light
theme.

White seems reasonable for dark theme, just like in light theme.
There shouldn't be an issue with contrast; I expect the background
will be darker, if anything, in dark theme.

We can always refine these styles later.

…t/dark

The dark-theme variant is copied from the web app.

We'll cover the other items one by one, for ease of review, next.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

The dark-theme variant is copied from the web app.

Web expresses this as separate `color` and `opacity` attributes. We
combine those attributes into a single Color, where the alpha is the
alpha from `color` multiplied by the `opacity`.

So, for light theme: 1 * 0.8 = 0.8
And for dark theme: 0.75 * 0.8 = 0.6

…ad hoc

This should have more reasonable contrast, but we need an approved
color from Vlad.