fix(v-text-field(box)): fix state colors

colors are now on border, updated selector

Author: johnleider

src/stylus/components/_text-fields.styl

@@ -337,4 +337,4 @@ rtl(v-text-field-rtl, "v-text-field")
 
   &.v-input--has-state
     .v-input__slot:before
-      background-color: currentColor
+      border-color: currentColor