Fix the midnight theme (#508)

* Revert "Remaster midnight theme for hierarchical styles"

This reverts commit c7a8fbb0.

* Clean up some of the colors in the midnight theme

- Add some colors for some extra Python tokens
- Make the current line easier to see
- Add back the link() calls from the reverted c7a8fbb0 (although I'm unsure
  what they do)

* Make the command line not bold in the midnight theme

* Use the default text color in the midnight theme

The theme claims to be best with green text, so we also use white instead of
green for the comment color.

* More cleanups to the midnight theme

- Make the highlighted line always blue instead of green
- Fix some color combinations that were hard to read
- Be consistent about highlighted and active things in the sidebar. Active is
  bold, highlighted is blue background, disabled is gray. In the variables
  view green (and lighter green for highlighted) is used for highlighted
  variables.
- Use lighter and darker yellow (actually brown and yellow) for the current
  line highlighted and not highlighted.
- Make the sidebars lighter when selected
- Make the hotkeys italics as well as underlined, since the underline is very
  hard to see

Some issues:

- The bold dark gray for disabled highlighted breakpoints is just showing as
  bold white. Don't know if that's a bug in my terminal or a bug in urwid, or
  if they really do just look the same.
- The hotkey letter in the highlighted pane does not change. Should there be a
  "focused hotkey" color?

* Fix flake8 issues

* Make some changes to the midnight theme from the review by @mvanderkamp

* Use light green as the main color for midnight instead of dark green

* Color operators in dark green in the midnight theme

* Some tweaks to the midnight theme based on review

* Get rid of some redundant settings in the midnight theme