Add nicer dark and light colour sets (themes) used on terminals with 256

or more colours. Currently based these on emacs but they could change.
Terminals with fewer colours use the ANSI colours. A new "theme" option
overrides the detected theme (set to "terminal" to go back to ANSI
colours).

tmux.1

@@ -4571,6 +4571,57 @@ where the number is a hexadecimal number, or a range of the form
 Give the command to pipe to if the
 .Ic copy\-pipe
 copy mode command is used without arguments.
+.It Xo Ic theme
+.Op Ic detect | terminal | light | dark
+.Xc
+Whether tmux should use the detected client theme
+.Pq Ic detect ,
+use terminal ANSI colours
+.Pq Ic terminal ,
+or force the light or dark theme.
+.It Xo Ic dark\-theme\-black ,
+.Ic dark\-theme\-white ,
+.Ic dark\-theme\-light\-grey ,
+.Ic dark\-theme\-dark\-grey ,
+.Ic dark\-theme\-green ,
+.Ic dark\-theme\-yellow ,
+.Ic dark\-theme\-red ,
+.Ic dark\-theme\-blue ,
+.Ic dark\-theme\-cyan ,
+.Ic dark\-theme\-magenta ,
+.Ic light\-theme\-black ,
+.Ic light\-theme\-white ,
+.Ic light\-theme\-light\-grey ,
+.Ic light\-theme\-dark\-grey ,
+.Ic light\-theme\-green ,
+.Ic light\-theme\-yellow ,
+.Ic light\-theme\-red ,
+.Ic light\-theme\-blue ,
+.Ic light\-theme\-cyan
+.No and Ic light\-theme\-magenta Ar colour
+.Xc
+Set the theme colours.
+These are the named colours
+.Ic themeblack ,
+.Ic themewhite ,
+.Ic themelightgrey ,
+.Ic themedarkgrey ,
+.Ic themegreen ,
+.Ic themeyellow ,
+.Ic themered ,
+.Ic themeblue ,
+.Ic themecyan
+and
+.Ic thememagenta
+which may be used in styles and elsewhere a
+.Ar colour
+is accepted.
+The
+.Ic dark\-theme\-*
+set is used when the client reports a dark background and the
+.Ic light\-theme\-*
+set when it reports a light background;
+an unknown background is treated as dark.
 .It Ic default\-client\-command Ar command
 Set the default command to run when tmux is called without a command.
 The default is