So that selection inside widgets isn't affected

Issue #3342

So that they don't end up overriding more specific styles.

Issue #3325

Matches color for `cm-link` style

Margins for gutter markers aren't supported, and this was
pushing the numbers into the content.

Issue #3098

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

Issue #3053

This file was taken from
https://github.com/wisenomad/zenburn-lighttable-theme with a few
modifications to make it work in CodeMirror

The color white is not visible on the current backgroung

It doesn't belong in a theme.

Changed background selected color to #303030, previous #202020 value is too dark.
https://github.com/cristatus/brackets-darker/issues/12

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

Closes #2415

Doesn't take up any space now, so that a tab of width 2 is 2 em width
rather than 3.

Added missing semicolon

`cm-s-ambiance` and `CodeMirror-focused` are classes to the same
element, so search for `.cm-s-ambiance .CodeMirror-focused` will fail.