Improve styling of thebe elements

[akhmerov]

After migration to the sphinx-book-theme, thebe elements look like this upon activation:

I think this would look better without line separators in the source code, and the button styling could be improved.

[akhmerov]

Also the code really doesn't look like CodeMirror abcdef theme...

[moorepants]

#184 is relevant. Maybe it needs to ship with minimal styling.

[stevejpurves]

this is now functional see https://executablebooks.github.io/thebe/ for darcula in place