[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

.gitignore

@@ -6,3 +6,4 @@
 /.sass-cache/
 /build/
 *.scssc
+/Book.html