Adjust base16 (16) theme (#4501)

Motivation:

`--color base16` 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.

src/options.go

@@ -59,7 +59,7 @@ Usage: fzf [options]
 
   GLOBAL STYLE
     --style=PRESET           Apply a style preset [default|minimal|full[:BORDER_STYLE]
-    --color=COLSPEC          Base scheme (dark|light|16|bw) and/or custom colors
+    --color=COLSPEC          Base scheme (dark|light|base16|bw) and/or custom colors
     --no-color               Disable colors
     --no-bold                Do not use bold text
 
@@ -1326,7 +1326,7 @@ func parseTheme(defaultTheme *tui.ColorTheme, str string) (*tui.ColorTheme, erro
 			theme = dupeTheme(tui.Dark256)
 		case "light":
 			theme = dupeTheme(tui.Light256)
-		case "16":
+		case "base16", "16":
 			theme = dupeTheme(tui.Default16)
 		case "bw", "no":
 			theme = tui.NoColorTheme()