A new theme for the kitty website

Supports dark mode. Also re-organize the documentation
a bit, making it more hierarchical.

Have a nicer landing page

docs/deccara.rst

@@ -0,0 +1,21 @@
+Setting text styles/colors in arbitrary regions of the screen
+------------------------------------------------------------------
+
+There already exists an escape code to set *some* text attributes in arbitrary
+regions of the screen, `DECCARA
+<https://vt100.net/docs/vt510-rm/DECCARA.html>`_.  However, it is limited to
+only a few attributes. |kitty| extends this to work with *all* SGR attributes.
+So, for example, this can be used to set the background color in an arbitrary
+region of the screen.
+
+The motivation for this extension is the various problems with the existing
+solution for erasing to background color, namely the *background color erase
+(bce)* capability. See
+`this discussion <https://github.com/kovidgoyal/kitty/issues/160#issuecomment-346470545>`_
+and `this FAQ <https://invisible-island.net/ncurses/ncurses.faq.html#bce_mismatches>`_
+for a summary of problems with *bce*.
+
+For example, to set the background color to blue in a
+rectangular region of the screen from (3, 4) to (10, 11), you use::
+
+    <ESC>[2*x<ESC>[4;3;11;10;44$r<ESC>[*x