Add support for themed img (#1445)

Closes #1405.

Author: yordis

assets/less/content.less

@@ -13,3 +13,7 @@
   @import './content/footer';
   @import './content/bottom-actions.less';
 }
+
+body:not(.night-mode) .content-inner img[src*="#gh-dark-mode-only"] {
+  display: none;
+}

assets/less/epub.less

@@ -17,6 +17,10 @@ nav > ol {
   }
 }
 
+img[src*="#gh-dark-mode-only"] {
+  display: none;
+}
+
 .title-container {
   text-align: center;
 }

assets/less/night/content/general.less

@@ -33,3 +33,7 @@ a.view-source {
 .summary h2 a {
   text-decoration: none;
 }
+
+img[src*="#gh-light-mode-only"] {
+  display: none;
+}