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.
The default theme is now chosen based on the user’s preference stored in
their device. If the device uses a dark theme, the website’s dark theme
will be used by default. Otherwise, the light theme will be used by
default.
