Also some general improvements to material.css and solarized.css

So that selection inside widgets isn't affected

Issue #3342

So that the contenteditable input style doesn't look silly
on desktop.

Code: Brackets now become black when they are highlighted as "matching".
JSON: key names are now in "white" (json was a bit saturated with orange - keys and string values).
Cleanup: removed some unnecessary rules.
Default face color normalized to #ffffec (was #ffffe9).

Closes #2623

Using the CodeMirror-guttermarker and CodeMirror-guttermarker-subtle CSS
classes to give the markers their color.

Issue #2602