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If you like the following result more than what is shown in the screenshot above, you only have to edit a single CSS file.

Here is what to do to change the font for your HipChat client:

1. Open your editor as admin
2. Open Program Files (x86)\Atlassian\HipChat4\localweb\hipchat-client.css
3. Remove the Helvetica fonts from the font family.

So here the The fragment of the CSS file containing the font-family to change:

 body,
#page,
#hipchat {
  font-family: Helvetica Neue, Helvetica, Arial, sans-serif;
  height: 100%;
  width: 100%;
  min-width: 750px;
  transition: 0.5s -webkit-filter linear, 0.5s filter linear;
}

 body,
#page,
#hipchat {
  font-family: Arial, sans-serif;
  height: 100%;
  width: 100%;
  min-width: 750px;
  transition: 0.5s -webkit-filter linear, 0.5s filter linear;
}

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