Fix disqus background in dark mode. (#732)

### Prerequisites

- [x] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

This PR fixes the bug about disqus iframe background in dark mode, it
also fix another bug when switching color mode.

Before:
<img width="881" alt="opaque background in dark mode"
src="https://user-images.githubusercontent.com/17041194/202862887-720dbb82-f0c8-4a24-97fd-78baef2a7cb4.png">

After:
<img width="883" alt="CleanShot 2022-11-20 at 01 12 52@2x"
src="https://user-images.githubusercontent.com/17041194/202863146-d5a246a7-bbe5-4bef-9687-883ce33f5b1a.png">

#### Why this happens

The problem is widely discussed
[here](https://github.com/w3c/csswg-drafts/issues/4772).

>If the color scheme of an iframe differs from embedding document,
iframe gets an opaque canvas bg appropriate to its color scheme.

So I made two fixes:

1. Add `color-scheme: light` CSS property for disqus `iframe`, this
creates a transparent background and fixes the opaque issue.
2. Add `themeChanged` event to notice disqus comments board refresh when
the theme change, this fixes the opaque problem when you switch the
mode.

#### Reference

I've been working on this issue for a whole night and the fixing idea
was strongly inspired by [Sergeyski's
Blog](https://sergeyski.com/css-color-scheme-and-iframes-lessons-learned-from-disqus-background-bug/).

### Issues Resolved

Fixes #722.