Set the default color for the search input (#516)

* Set the default color for the search input

Since the search input text color is not set, if a user has changed its default color (ie. when using a dark theme), then the search input text can be unreadable.

* Update search color to $dark

Author: AlexandreBonneau

themes/vue/source/css/_header.styl

@@ -42,6 +42,7 @@ body.docs
     box-sizing border-box
     padding 0 15px 0 30px
     border 1px solid #e3e3e3
+    color $dark
     outline none
     border-radius 15px
     margin-right 10px