Let the header and footer follow the main column

With keep_columns and empty side columns the posts sit in a centred
column while the header and footer still run edge to edge, which reads as
two different pages stacked. The new switch insets their content by the
column widths (the backgrounds stay full width, phones are untouched) so
the whole page lines up as one column — the look a single-column site
expects, without bringing back a content max-width.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

1 files changed, 2 insertions, 1 deletions

diff --git a/README.md b/README.md
index 90dca7a..df35621 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,8 @@ Server-rendered HTML, no JavaScript required, works on old browsers and phones.
   tags (with counts), tag cloud, a search box (finds posts by title and text, any
   case; several words match in order with anything in between), custom HTML, footer
   text, RSS link, site map. Column widths are percentages; on phones the columns
-  stack under the posts. The **menu** mixes blog pages and outside links. Posts per
+  stack under the posts; the header and footer can follow the main column's edges
+  for a centred, single-column look. The **menu** mixes blog pages and outside links. Posts per
   page, the date format, the site title size and the footer alignment live here too,
   as does whether listings show whole posts or **summaries** with a "Read more" link
   (a post's summary ends at a `<!--more-->` line, or after its first paragraphs).