Add a "how to" on creating a custom theme and theming options

[willingc]

It would be nice to highlight the possibilities and what extension points exist.

[nthakor]

Is there any place where this documentation exists in any form? Especially regarding fonts.

[7k50]

Is there anywhere custom theming can be done – and if so, where are the files located?

~/.jupyter/nteract.json has one row in it: {"theme":"dark"}. I'm imagining that if I can find that "one-dark" theme I might be able to edit it. But I'm not able to find where it's located on my computer (I'm using the nteract Desktop Mac app).