add background support.

Readme.org

@@ -178,6 +178,64 @@ $ git clone https://github.com/yjwen/org-reveal.git
 
     Available transitions are: default|cube|page|concave|zoom|linear|fade|none.
 
+** Set Slide Background
+
+   Slide background can be set to a color, an image or a repeating image
+   array by setting heading properties.
+
+*** Single Colored Background
+   :PROPERTIES:
+   :reveal_background: #543210
+   :END:
+
+    Set property =reveal_background= to either an RGB color value, or any
+    supported CSS color format.
+
+#+BEGIN_SRC org
+,*** Single Colored Background
+   :PROPERTIES:
+   :reveal_background: #123456
+   :END:
+#+END_SRC
+
+*** Single Image Background
+    :PROPERTIES:
+    :reveal_background: ./images/whale.jpg
+    :reveal_background_trans: slide
+    :END:
+
+    Set property =reveal_background= to an URL of background image.
+    Set property =reveal_background_trans= to =slide= to make background image
+    sliding rather than fading.
+#+BEGIN_SRC org
+,*** Single Image Background
+    :PROPERTIES:
+    :reveal_background: ./images/whale.jpg
+    :reveal_background_trans: slide
+    :END:
+#+END_SRC
+
+*** Repeating Image Background
+    :PROPERTIES:
+    :reveal_background: ./images/whale.jpg
+    :reveal_background_size: 200px
+    :reveal_background_repeat: repeat
+    :END:
+
+    Resize background image by setting property
+ =reveal_background_size= to a number.
+
+    Set property =reveal_background_repeat= to =repeat= to repeat
+    image on the background.
+#+BEGIN_SRC org
+,*** Repeating Image Background
+    :PROPERTIES:
+    :reveal_background: ./images/whale.jpg
+    :reveal_background_size: 200px
+    :reveal_background_repeat: repeat
+    :END:
+#+END_SRC
+
 ** Slide Size
 
    Reveal.js scales slides to best fit the display resolution. But in case

ox-reveal.el

@@ -277,12 +277,16 @@ holding contextual information."
              ;; into vertical ones.
              "<section>\n")
          ;; Start a new slide.
-         (format "<section id=\"%s\" %s>\n"
+         (format "<section id=\"%s\" %s%s%s%s%s>\n"
                  (or (org-element-property :CUSTOM_ID headline)
                      (concat "sec-" (mapconcat 'number-to-string
                                                (org-export-get-headline-number headline info)
                                                "-")))
-                 (if-format " data-state=\"%s\"" (org-element-property :REVEAL_DATA_STATE headline)))
+                 (if-format " data-state=\"%s\"" (org-element-property :REVEAL_DATA_STATE headline))
+                 (if-format " data-background=\"%s\"" (org-element-property :REVEAL_BACKGROUND headline))
+                 (if-format " data-background-size=\"%s\"" (org-element-property :REVEAL_BACKGROUND_SIZE headline))
+                 (if-format " data-background-repeat=\"%s\"" (org-element-property :REVEAL_BACKGROUND_REPEAT headline))
+                 (if-format " data-background-transition=\"%s\"" (org-element-property :REVEAL_BACKGROUND_TRANS headline)))
          ;; The HTML content of this headline.
          (format "\n<h%d%s>%s</h%d>\n"
                  level1