Add border when tree view is mounted on the right
Add border for panes

Use same border as default Atom UI theme to show separation of panes
Add background color to gutter

Matches Xcode’s gutter for line numbers
Revert gutter background

This belongs in syntax not UI theme.
Better color to match sidebar and tab borders.
Better tab styles when columns are in play.
Merge v0.3.0 from upstream