Update help, man page and completions

doc/long-help.txt

@@ -109,10 +109,18 @@ Options:
             'bat --ignored-suffix ".dev" my_file.json.dev' will use JSON syntax, and ignore '.dev'
 
       --theme <theme>
-          Set the theme for syntax highlighting. Note that this option overrides '--theme-dark' and
-          '--theme-light'. Use '--list-themes' to see all available themes. To set a default theme,
-          add the '--theme="..."' option to the configuration file or export the BAT_THEME
-          environment variable (e.g.: export BAT_THEME="...").
+          Set the theme for syntax highlighting. Use '--list-themes' to see all available themes. To
+          set a default theme, add the '--theme="..."' option to the configuration file or export
+          the BAT_THEME environment variable (e.g.: export BAT_THEME="...").
+          
+          Special values:
+          
+            * auto: Picks a dark or light theme depending on the terminal's colors (default).
+                    Use '--theme-light' and '--theme-dark' to customize the selected theme.
+              * auto:always: Detect the terminal's colors even when the output is redirected.
+              * auto:system: Detect the color scheme from the system-wide preference (macOS only).
+            * dark: Use the dark theme specified by '--theme-dark'.
+            * light: Use the light theme specified by '--theme-light'.
 
       --theme-light <theme>
           Sets the theme name for syntax highlighting used when the terminal uses a light