Add dark theme

[itamaro]

Most of the time I'm in Sublime Text, with the beautiful dark Molokai theme.

It would be really nice to be able to configure TodoTxtMac with a similar theme, so it isn't so bright every time I switch to it.

[mjdescy]

I would like this too, but I think the UI needs a rewrite to make this work well. Currently it is using cell-based tables instead of view-based tables. Everything needs to be re-linked to make things more modern.

[scottmmjackson]

See #130 for some work on this.