[Style] Add Nord theme, break up SCSS code

This commit adds the Nord theme into the list of available themes. The
Nord theme is now the default theme for dark-based systems.

Theme buttons now have their own class to make it easier to change
them without too much work.

The `style.scss' file has been split up in three different files:
- main.scss, the new file to load from the HTML page, imports the two
    other files
- themes.scss, containing all the theming of the websites
- style.scss, containing the rest

All websites loading the theme will now need to load it from
`main.scss' instead of `style.scss', likewise with the `.css'
extension

web/dart/navbar.dart

@@ -114,7 +114,9 @@ Future<Element> makeThemeChanger() async {
   Element makeThemeItem(String t_btnId) {
     return Element.li()
       ..classes.add('dropdown-item')
-      ..append(Element.span()..attributes['id'] = t_btnId);
+      ..append(Element.span()
+        ..attributes['id'] = t_btnId
+        ..classes.add('themeBtn'));
   }
 
   return Element.li()
@@ -131,6 +133,7 @@ Future<Element> makeThemeChanger() async {
       ..attributes['id'] = 'theme-dropdown'
       ..append(makeThemeItem('lightBtn'))
       ..append(makeThemeItem('darkBtn'))
+      ..append(makeThemeItem('nordBtn'))
       ..append(makeThemeItem('blackBtn')));
 }