How to Remove Background in Google Hangouts

@Rubén's answer is correct, given the question, but I'd like to slightly improve it with a minor nit:

.kFx1Ae-xdwExf-eb-m, .Id-qy-By {
    display:none;
}

The additional class, Id-qy-By, targets acknowledgements displayed along with photographs retrieved from Google+, such as:

Photo by Alex Wrigley

shared on Google+

(sample screenshot containing above notice)

Note that photos selected from internal repository (gstatic servers) don't include such acknowledgements.

Bottom line is: No photo... No credit! ;-)

P.S.: I'd comment instead of proposing a separate answer but not enough reputation was available at the time of this writing.

Update 2017-01-03: Full rule being used in Stylish (2.0.7 for Firefox 50.1.0), which features tighter filtering for improved performance: domain (hangouts.google.com) and element type (div):

@-moz-document domain("hangouts.google.com") {
  div.kFx1Ae-xdwExf-eb-m, div.Id-qy-By {
    display:none;
  }
}

Update 2017-11-08: A similar procedure works for Stylus (1.1.4.2 for Firefox 56), now that Stylish is (currently) marked legacy:

1. Create a new rule
2. In Mozilla Format, press the Import button
3. Paste the code from the section above
4. Name the rule (for example, Hangout hide background element)

Update 2018-11-05: CSS class updated after noticing Hangouts back-end upgrade (which changed some infrastructure and, therefore, broke the previous rules for the photo itself):

@-moz-document domain("hangouts.google.com") {
  div.g-Qx-eb, div.Id-qy-By {
    display:none;
  }
}