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.

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

Issue #2602

Added missing semicolon

Rather than adding a new rule.

Issue #1747

Closes #1744