css: copy basic style from the libgit2 project

Seeing the color-scheme is fairly normal around most web pages related
to Git (git-scm.com and libgit2), we can use it here as well.

Based on the original, tweak blockquotes and a few other things to look
nicer, particularly for rev_news.

Also include libgit2's note on copying/license.

Author: tfnico

css/COPYING

@@ -0,0 +1,24 @@
+Most of the content from the main.css file has been copied from the
+https://github.com/libgit2/libgit2.github.com project, which uses
+the following license:
+
+Copyright (c) 2013 The libgit2 contributors
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.