Simplify code, remove html dependency

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.

pubspec.yaml

@@ -8,9 +8,8 @@ environment:
   sdk: '>=2.7.0 <3.0.0'
 
 dependencies:
-  html: '^0.14.0+3'
 
 dev_dependencies:
-  build_runner: ^1.8.0
+  build_runner: ^1.10.1
   build_web_compilers: ^2.9.0
   pedantic: ^1.9.0