fix highlight color (vuejs#271)

Author: sdras

src/.vuepress/theme/styles/code.styl

@@ -41,7 +41,7 @@ div[class*="language-"]
     width 100%
     line-height 1.4
     .highlighted
-      background-color rgb(248 236 204 / 90%)
+      background-color #f3edd1
   pre, pre[class*="language-"]
     background transparent
     position relative
@@ -68,7 +68,7 @@ div[class*="language-"]
           display block
           width $lineNumbersWrapperWidth
           height 100%
-          background-color rgba(255, 255, 255, 95%)
+          background-color #f3edd1
     pre
       padding-left $lineNumbersWrapperWidth + 1 rem
       vertical-align middle