[SCSS, some dart] Theme rework, dark dropped, nord replaces dark

The old dark mode is dropped and replaced with nord. Light and black
themes were also rebased on the Nord theme.

Code blocks now appear the same way my terminal windows appear in my
Linux configuration, emulating a side titlebar.

web/dart/navbar.dart

@@ -131,7 +131,6 @@ Future<Element> makeThemeChanger() async {
       ..attributes['id'] = 'theme-dropdown'
       ..append(makeThemeItem('lightBtn'))
       ..append(makeThemeItem('darkBtn'))
-      ..append(makeThemeItem('nordBtn'))
       ..append(makeThemeItem('blackBtn')));
 }