Implement the dark/light theme switcher.

[thomasf]

[skitapa]

Thank you for an awesome addon to our favorite org-mode, I was mighty glad when I found it 👍. Would be great with a theme switcher. I send a monthly journal to my boss and because I don't know if he prefers light or dark; a switcher would be great 😄

[i5heu]

This would be still cool!