Change html theme again, fix code blocks

The agogo theme doesn't do very well on narrow screens. The code
blocks were used for shell commands but were using Python syntax
highlighting.

manual/cli.rst

@@ -13,7 +13,7 @@ Basic Invocation
 
 When running qpdf, the basic invocation is as follows:
 
-::
+.. code-block:: text
 
    qpdf [ options ] { infilename | --empty } outfilename
 
@@ -456,7 +456,7 @@ Encryption Options
 To change the encryption parameters of a file, use the --encrypt flag.
 The syntax is
 
-::
+.. code-block:: text
 
    --encrypt user-password owner-password key-length [ restrictions ] --
 
@@ -620,7 +620,7 @@ selecting pages from one or more input files. Whatever file is given as
 the primary input file is used as the starting point, but its pages are
 replaced with pages as specified.
 
-::
+.. code-block:: text
 
    --pages input-file [ --password=password ] [ page-range ] [ ... ] --
 
@@ -761,7 +761,7 @@ copying all of that from the first file. For example, to take pages 1
 through 5 from a :file:`infile.pdf` while preserving
 all metadata associated with that file, you could use
 
-::
+.. code-block:: text
 
    qpdf infile.pdf --pages . 1-5 -- outfile.pdf
 
@@ -769,7 +769,7 @@ If you wanted pages 1 through 5 from
 :file:`infile.pdf` but you wanted the rest of the
 metadata to be dropped, you could instead run
 
-::
+.. code-block:: text
 
    qpdf --empty --pages infile.pdf 1-5 -- outfile.pdf
 
@@ -778,7 +778,7 @@ If you wanted to take pages 1 through 5 from
 :file:`file2.pdf` in reverse, taking document-level
 metadata from :file:`file2.pdf`, you would run
 
-::
+.. code-block:: text
 
    qpdf file2.pdf --pages file1.pdf 1-5 . 15-11 -- outfile.pdf
 
@@ -787,10 +787,12 @@ file called :file:`encrypted.pdf` with password
 ``pass`` and repeat it twice in an output file, and if you wanted to
 drop document-level metadata but preserve encryption, you would use
 
-::
+.. code-block:: text
 
-   qpdf --empty --copy-encryption=encrypted.pdf --encryption-file-password=pass
-   --pages encrypted.pdf --password=pass 1 ./encrypted.pdf --password=pass 1 --
+   qpdf --empty --copy-encryption=encrypted.pdf \
+        --encryption-file-password=pass \
+        --pages encrypted.pdf --password=pass 1 \
+              ./encrypted.pdf --password=pass 1 -- \
         outfile.pdf
 
 Note that we had to specify the password all three times because giving
@@ -823,7 +825,7 @@ Starting with qpdf 8.4, it is possible to overlay or underlay pages from
 other files onto the output generated by qpdf. Specify overlay or
 underlay as follows:
 
-::
+.. code-block:: text
 
    { --overlay | --underlay } file [ options ] --
 

manual/conf.py

@@ -11,7 +11,7 @@ project = 'QPDF'
 copyright = '2005-2021, Jay Berkenbilt'
 author = 'Jay Berkenbilt'
 release = '10.4.0'
-html_theme = 'agogo'
+html_theme = 'nature'
 html_theme_options = {
     "body_max_width": None,
 }

manual/installation.rst

@@ -75,7 +75,7 @@ Build Instructions
 
 Building qpdf on UNIX is generally just a matter of running
 
-::
+.. code-block:: text
 
    ./configure
    make