Add dark mode

[AnceretMatthieu]

This pull request add a dark mode to this theme (like in my fork, we can see a demo on my blog).

Note :

• I change blue link color by orange (it makes better on dark background). If you want, i can make it configurable.
• I add a toggle to switch between light and dark mode (by default, set to dark). If you want, i can make default mode configurable and add the possibility to disable/hide the toggle.
• All colors are in src\scss\utils\mixins_themed.scss file. The system is designed to allow the addition of additional mode.

If you have questions or feedback I'm happy to discuss.

Matthieu.

[kakawait]

Thank you so much. As discussed I'll review as soon I'll come back.

Plan for 0.6.0, just have to finish 0.5.2 but is patch version should not affect / conflict with your PR

[kakawait]

Globally is nice.

I've some small changes to do but I'll merge on develop then will fix upon develop

What I'd like to changes:

• Put script outside head_end.html, head_end.html is a hook to let end user customize head.html it must stay "blank"
• Some pieces of coding styles

For CSS there is too much changes I can't compile that on my head haha. I'll see more on live