Rename theme 'highlight' to 'dark'

Leave space for future 'light' theme.

README.rst

@@ -12,7 +12,7 @@ python >= 3.3 and ``less`` as a pager.
 
 See screenshots below.
 
-Theme ``default``:
+Theme ``default`` (works well for both dark and light terminal background):
 
 .. image:: https://github.com/ymattw/ydiff/blob/master/img/side-by-side-default.png
    :alt: side by side, theme 'default'
@@ -24,15 +24,15 @@ Theme ``default``:
    :align: center
    :height: 300 px
 
-Theme ``highlight``:
+Theme ``dark``: (for dark terminal background):
 
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/side-by-side-highlight.png
-   :alt: side by side, theme 'highlight'
+.. image:: https://github.com/ymattw/ydiff/blob/master/img/side-by-side-dark.png
+   :alt: side by side, theme 'dark'
    :align: center
    :height: 300 px
 
-.. image:: https://github.com/ymattw/ydiff/blob/master/img/unified-highlight.png
-   :alt: unified, theme 'highlight'
+.. image:: https://github.com/ymattw/ydiff/blob/master/img/unified-dark.png
+   :alt: unified, theme 'dark'
    :align: center
    :height: 300 px
 
@@ -157,8 +157,7 @@ Type ``ydiff -h`` to show usage::
                             pager application to feed output to, default is 'less'
       -o OPT, --pager-options=OPT
                             options to supply to pager application
-      --theme=THEME         option to pick a color theme (one of default,
-                            highlight)
+      --theme=THEME         option to pick a color theme (one of default, dark)
 
       Note:
         Option parser will stop on first unknown option and pass them down to

ydiff.py

@@ -70,7 +70,7 @@ class Color(object):
         'file_separator': (Color.FG_BRIGHT_CYAN,),
         'wrap_marker': (Color.FG_BRIGHT_MAGENTA,),
     },
-    'highlight': {
+    'dark': {
         # kind: (effects,)
         'header': (Color.FG_CYAN,),
         'old_path': (Color.BG8_DARK_RED,),
@@ -789,7 +789,7 @@ def _process_args(self, largs, rargs, values):
     parser.add_option(
         '-o', '--pager-options', metavar='OPT',
         help="""options to supply to pager application""")
-    themes = ', '.join(sorted(THEMES.keys()))
+    themes = ', '.join(['default', *sorted(THEMES.keys() - {'default'})])
     parser.add_option(
         '', '--theme', metavar='THEME', default='default',
         help="""option to pick a color theme (one of %s)""" % themes)