Allow a theme file as argument to --highlight-style.

Also include a sample, `default.theme`, in `data/`.

MANUAL.txt

@@ -630,7 +630,7 @@ General writer options
 :   Disables syntax highlighting for code blocks and inlines, even when
     a language attribute is given.
 
-`--highlight-style=`*STYLE*
+`--highlight-style=`*STYLE*|*FILE*
 
 :   Specifies the coloring style to be used in highlighted source code.
     Options are `pygments` (the default), `kate`, `monochrome`,
@@ -639,6 +639,12 @@ General writer options
     [Syntax highlighting], below.  See also
     `--list-highlight-styles`.
 
+    Instead of a *STYLE* name, a JSON file with extension
+    `.theme` may be supplied.  This will be parsed as a KDE
+    syntax highlighting theme and (if valid) used as the
+    highlighting style.  To see a sample theme that can be
+    modified, `pandoc --print-default-data-file default.theme`.
+
 `--syntax-definition=`*FILE*
 
 :   Instructs pandoc to load a KDE XML syntax definition file,

pandoc.cabal

@@ -109,6 +109,8 @@ Data-Files:
                  data/sample.lua
                  -- pandoc lua module
                  data/pandoc.lua
+                 -- sample highlighting theme
+                 data/default.theme
                  -- bash completion template
                  data/bash_completion.tpl
                  -- jats csl

src/Text/Pandoc/App.hs

@@ -56,7 +56,7 @@ import Data.Yaml (decode)
 import qualified Data.Yaml as Yaml
 import Network.URI (URI (..), isURI, parseURI)
 import Paths_pandoc (getDataDir)
-import Skylighting (Style, Syntax (..), defaultSyntaxMap)
+import Skylighting (Style, Syntax (..), defaultSyntaxMap, parseTheme)
 import Skylighting.Parser (missingIncludes, parseSyntaxDefinition,
                            addSyntaxDefinition)
 import System.Console.GetOpt
@@ -802,7 +802,13 @@ writerFn f   = liftIO . UTF8.writeFile f
 
 lookupHighlightStyle :: Maybe String -> IO (Maybe Style)
 lookupHighlightStyle Nothing = return Nothing
-lookupHighlightStyle (Just s) =
+lookupHighlightStyle (Just s)
+  | takeExtension s == ".theme" = -- attempt to load KDE theme
+    do contents <- B.readFile s
+       case parseTheme contents of
+            Left _    -> err 69 $ "Could not read highlighting theme " ++ s
+            Right sty -> return (Just sty)
+  | otherwise =
   case lookup (map toLower s) highlightingStyles of
        Just sty -> return (Just sty)
        Nothing  -> err 68 $ "Unknown highlight-style " ++ s