Document how to achieve inverted 1-bit color depth styling

[neiljp]

With #563, monochrome support is much improved, for a truly high-contrast (or retro!) experience.

However:

• Can we improve the 'colors' used for each style? There are very few combinations available, of course!
• Currently in 1-bit (monochrome) color-depth, every theme is identical, as having many 'themes' makes little sense in this context; however, we could have a 'light' (inverted) style, perhaps?

[neiljp]

I'm going to narrow this issue's focus since:

• the monochrome styling is already very usable now
• we could always slightly improve the monochrome styling, but this doesn't deserve an issue
• the context here is really the behavior in 1-bit color depth, which is subtly different from a monochrome theme!

Therefore the remaining significant element(s) would be to investigate achieving an inverted 1-bit behavior:

• document how this can be already be achieved if the terminal itself has inverted colors (often a terminal setting)
• (possibly as a follow-up) implement a specific setting for 1-bit color depth with an alternate inverted behavior

This is likely a good first issue at this point, but please discuss on the zulip-terminal stream on chat.zulip.org first.