fix: Coverpage when content > viewport height

fix #381

Author: jhildenbiddle

src/themes/basic/_coverpage.styl

@@ -1,10 +1,11 @@
 section.cover
+  position relative
   align-items center
   background-position center center
   background-repeat no-repeat
   background-size cover
-  height 100vh
-  width 100vw
+  min-height 100vh
+  width 100%
   display none
 
   &.show
@@ -15,12 +16,12 @@ section.cover
     opacity 0.8
     position absolute
     top 0
-    height 100%
+    bottom 0
     width 100%
 
   .cover-main
     flex 1
-    margin -20px 16px 0
+    margin 0 16px
     text-align center
     position: relative