This commit removes the `html' package dependency, speeding up the
compile time of the Dart code.
It also simplifies the code and removes some unused code. For instance
the sun, moon, and lightbulb icons is removed, the `Theme' class is
removed, and the `switchTheme' function is now merged into the
`setTheme' function. The `makeThemeItem' function has also had its
second argument removed.
Table of content was made unavailable regardless of whether one was
provided by the page or not. This commit fixes this issue by making an
unavailable TOC only if one is not provided by the web page, otherwise
the TOC will be available.
