Support Dark Mode

[arai-a]

This adds dark mode for the spec.

The colors and the variable names are chosen without much consideration, and I'm open to any changes.

There are some issue with the transparent images, such as the ECMA logo.
If there's dark-mode logo or any other solution, let me know.
For other images in figure, I've added always-#fff background color.

[michaelficarra]

We don't have a dark-mode Ecma logo, so for now that'll have to have a light background applied to it as well. My initial reaction is that the main document background is way too dark, creating eye-searing contrast with the body text. Why isn't it a similar tone to the left and bottom panes?

I'd like to see what an accessibility analysis says about this. Can you run Lighthouse (or some other accessibility tool) on the result, or host it on gh-pages so we can do it ourselves?

[arai-a]

We don't have a dark-mode Ecma logo, so for now that'll have to have a light background applied to it as well.

Okay, I've added #fff background to it as well.

My initial reaction is that the main document background is way too dark, creating eye-searing contrast with the body text. Why isn't it a similar tone to the left and bottom panes?

The color was just the opposite of the original color/background-color.
I've made the contrast almost same as GitHub's main text.

I'd like to see what an accessibility analysis says about this. Can you run Lighthouse (or some other accessibility tool) on the result, or host it on gh-pages so we can do it ourselves?

Pushed the output to https://arai-a.github.io/ecmarkup/out/

I'll check the accessibility details.