feat: introduce dual themes, make themes easier to apply

[MarkoSagadin]

Several CSS theme changes in these one:

• frontend build system now generates dual (light/dark) themes that are easy to switch between in the Anki clients, by changing the theme preference settings.
• The individual single color themes are still available for theme_builder purposes (theme inspection and development in the web browser).
• Each theme now also contains the CSS code from themeless_main.css file. That way users can just copy-paste one of the CSS themes in themes folder and apply it directly in Anki. Previously, if someone did that, the theme would not get applied due to the missing themeless_main.cssj.
• Selector :root.nightMode was changed to :root in all CSS files: the latter is the only one that works in both Anki and web browser.
• Documentation was updated to reflect the updated state.

Closes: #33