feat: add <kbd> styles (#60)

Author: slice

lib/default-theme/styles/theme.styl

@@ -71,6 +71,13 @@ p a code
   font-weight 400
   color $accentColor
 
+kbd
+  background #eee
+  border solid 0.15rem #ddd
+  border-bottom solid 0.25rem #ddd
+  border-radius 0.15rem
+  padding 0 0.15em
+
 blockquote
   font-size 1.2rem
   color #999
@@ -120,7 +127,7 @@ a.header-anchor
   &:hover
     text-decoration none
 
-code
+code, kbd
   font-family source-code-pro, Menlo, Monaco, Consolas, "Courier New", monospace
 
 p, ul, ol