Dark Theme added :)

[deisygysi]

Included the toggle button with day/night symbol and theme colours were adapted to dark mode.

Some themes need further check on the colours for the dark themes, the colours are not optimal and do not necessarily fit into Tachyons palletes.

I also added math support for the markdown.

Thanks,
Deisy

[kketevani]

Hello Deisy, thank you very much for sharing "dark theme mode". I would like to incorporate it on my website which is built by Hugo Apero. I followed your files and tracked all changes. I did all the same changes to my files. however, unfortunately, I see only the button, the dark mode is not available when I try to switch. maybe you would have a clue what may be the reason for that? thank you very much!