Web: fix column headings that lack gradients (issue 203050043 by address@hidden)

[paulwmorris]

Reviewers: ,

Message:
Please review, thanks.

Description:
Web: fix column headings that lack gradients

On the following two webpages there are
headings that are solid colors rather than
gradients, and this looks broken.
  introduction/freedom
  download

This patch adds two CSS classes for this
kind of heading, and removes the classes
that create solid color headings.

Includes three commits:

Web: use column-center-middle- CSS classes
use column-center-middle-color2 and
column-center-middle-color3 instead of
div.color2, div.color3, etc.

Web: CSS: add column-center-middle classes
add column-center-middle-color2
add column-center-middle-color3
remove div.color1 .color2 .color3 .color4

Web: CSS: put column h3 headers together

Please review this at https://codereview.appspot.com/203050043/

Affected files (+78, -101 lines):
  M Documentation/css/lilypond-website.css
  M Documentation/web/download.itexi
  M Documentation/web/introduction.itexi