Added CSS Markdown class for full width background gradient header im…

…age.

docs/assets/css/theme.css

@@ -405,10 +405,20 @@
   z-index: -1;
 }
 
-@media screen and (max-width: 980px) {
-  .markdown-section .header-image-fade {
-    top: -20px;
-  }
+/* CSS currently only works with sidebar and standalone pages */
+.markdown-section .header-image-fade-full-width {
+  margin-top: -2rem;
+  left: 50%;
+  right: 50%;
+  margin-left: -50vw;
+  margin-right: -50vw;
+  position: absolute;
+  width: 100vw;
+  max-width: 100vw;
+  -webkit-mask-image: -webkit-gradient(linear, left top, left bottom, from(rgba(0,0,0,1)), to(rgba(0,0,0,0)));
+  mask-image: linear-gradient(to bottom, rgba(0,0,0,1), rgba(0,0,0,0));
+  opacity: 0.4;
+  z-index: -1;
 }
 
 /* CSS currently only works with sidebar and standalone pages */