Adjust default themes

Motivation:

`--color 16` can be a better default than `dark` or `light`, since it uses
the colors defined by the current theme. This usually blends in more
naturally and works well in both light and dark modes.

However, some elements were previously hard-coded with white or black
foreground colors, which can cause rendering issues in certain terminal
themes.

* 16
  * Avoid using black or white foreground colors, so it works better with
    both dark and light themes
  * Display 'info' in italic to better separate it from the other parts
* dark / light
  * Display 'info' in italic for consistency

src/tui/tui.go

@@ -873,13 +873,13 @@ func init() {
 		Current:          ColorAttr{colYellow, AttrUndefined},
 		CurrentMatch:     ColorAttr{colGreen, AttrUndefined},
 		Spinner:          ColorAttr{colGreen, AttrUndefined},
-		Info:             ColorAttr{colWhite, AttrUndefined},
+		Info:             ColorAttr{colDefault, Italic},
 		Cursor:           ColorAttr{colRed, AttrUndefined},
 		Marker:           ColorAttr{colMagenta, AttrUndefined},
 		Header:           ColorAttr{colCyan, AttrUndefined},
 		Footer:           ColorAttr{colCyan, AttrUndefined},
-		Border:           ColorAttr{colBlack, AttrUndefined},
+		Border:           ColorAttr{colDefault, Dim},
-		BorderLabel:      ColorAttr{colWhite, AttrUndefined},
+		BorderLabel:      ColorAttr{colDefault, AttrUndefined},
 		Ghost:            ColorAttr{colUndefined, Dim},
 		Disabled:         ColorAttr{colUndefined, AttrUndefined},
 		PreviewFg:        ColorAttr{colUndefined, AttrUndefined},
@@ -921,7 +921,7 @@ func init() {
 		Current:          ColorAttr{254, AttrUndefined},
 		CurrentMatch:     ColorAttr{151, AttrUndefined},
 		Spinner:          ColorAttr{148, AttrUndefined},
-		Info:             ColorAttr{144, AttrUndefined},
+		Info:             ColorAttr{144, Italic},
 		Cursor:           ColorAttr{161, AttrUndefined},
 		Marker:           ColorAttr{168, AttrUndefined},
 		Header:           ColorAttr{109, AttrUndefined},
@@ -969,7 +969,7 @@ func init() {
 		Current:          ColorAttr{237, AttrUndefined},
 		CurrentMatch:     ColorAttr{23, AttrUndefined},
 		Spinner:          ColorAttr{65, AttrUndefined},
-		Info:             ColorAttr{101, AttrUndefined},
+		Info:             ColorAttr{101, Italic},
 		Cursor:           ColorAttr{161, AttrUndefined},
 		Marker:           ColorAttr{168, AttrUndefined},
 		Header:           ColorAttr{31, AttrUndefined},