feat: enhance the styles of blockquote

Author: ulivz

packages/@vuepress/theme-default/styles/index.styl

@@ -94,10 +94,10 @@ kbd
   padding 0 0.15em
 
 blockquote
-  font-size .9rem
-  color #999
-  border-left .5rem solid #dfe2e5
-  margin 0.5rem 0
+  font-size 1rem
+  color #999;
+  border-left .2rem solid #dfe2e5
+  margin 1rem 0
   padding .25rem 0 .25rem 1rem
 
   & > p