Inconsistent syles of markdown editor

Bug

• Inconsistent selected text styles
• Toolbar buttons missing "active" styles
• The current font is hard to read and there is no need for it ( monospace )
• Dark theme for markdown editor #4154
• Reduce the need for toggling preview mode.
• Add More consistency of styles between edit and preview mode.

To Reproduce Steps to reproduce the behavior:

1. Go to 'Publish area'
2. Use the markdown editor

Expected behavior Consistent styles and colors for all themes.