Highlight in Elasticsearch
As I stated in the comments I don't think this can be done in Elasticsearch. A highlighter just highlights the terms it matched and does no further postprocessing (And I found no evidence in the docs for Elasticsearch 2.3 that you could make it able to do so).
Anyway, my logical approach would be to add the <b>...</b> tags when you're rendering the HTML code.
{{ foreach hit in hits }}<b>...</b> hit[content] <b>...</b>{{ endfor }}Something like this, just modify it to suit your template.
Use pre_tags and post_tags for this purpose.see configuring tags
GET /_search{ "query" : { "match": { "user": "kimchy" } }, "highlight" : { "pre_tags" : ["<b>"], "post_tags" : ["</b>"], "fields" : { "content" : {} } }}