I've been using this userstyle for a long time, and I wanted to share it since it still works with Firefox Quantum.

/*
 * Originally from:
 * http://forums.mozillazine.org/viewtopic.php?p=14404121#p14404121
 */

#urlbar {
  position: relative;
  z-index: 1;
}
#identity-box:after {
  content: '';
  position: absolute;
  height: 100%;
  width: 100%;
  top: 0;
  left: 0;
  pointer-events: none;
  z-index: -1;
  background: white;
  opacity: 0.2;
}

/* There is also grantedPermissions, but irrelevant. */

/* about:about */
#urlbar[pageproxystate='valid'] #identity-box.unknownIdentity:after {
  background: #ff0039; /* Firefox Red 50 */
}
/* about:config */
#urlbar[pageproxystate='valid'] #identity-box.chromeUI:after {
  background: #0a84ff; /* Firefox Blue 50 */
}
/* uBlock Origin Dashboard */
#urlbar[pageproxystate='valid'] #identity-box.extensionPage:after {
  background: #45a1ff; /* Firefox Blue 40 */
}
/* https://www.github.com/ */
#urlbar[pageproxystate='valid'] #identity-box.verifiedIdentity:after{
  background: #058b00; /* Firefox Green 70 */
}
/* https://www.google.com/ */
#urlbar[pageproxystate='valid'] #identity-box.verifiedDomain:after{
  background: #12bc00; /* Firefox Green 60 */
}
/* https://mixed-script.badssl.com/ */
#urlbar[pageproxystate='valid'] #identity-box.mixedActiveBlocked:after {
  background: #30e60b; /* Firefox Green 50 */
}
/* https://mixed.badssl.com/ */
#urlbar[pageproxystate='valid'] #identity-box.mixedDisplayContent:after {
  background: #d7b600; /* Firefox Yellow 60 */
}
/* https://very.badssl.com/ */
#urlbar[pageproxystate='valid'] #identity-box.mixedDisplayContentLoadedActiveBlocked:after {
  background: #d7b600; /* Firefox Yellow 60 */
}
/* https://self-signed.badssl.com/ but add certificate exception */
#urlbar[pageproxystate='valid'] #identity-box.certUserOverridden:after {
  background: #ffe900; /* Firefox Yellow 50 */
}
/* Don't know an example for this */
#urlbar[pageproxystate='valid'] #identity-box.weakCipher:after {
  background: #a47f00; /* Firefox Yellow 70 */
}
/* https://mixed-script.badssl.com/ but disable protection */
#urlbar[pageproxystate='valid'] #identity-box.mixedActiveContent:after {
  background: #d70022;  /* Firefox Red 60 */
}
/* http://http-login.badssl.com/ */
#urlbar[pageproxystate='valid'] #identity-box.insecureLoginForms:after {
  background: #a4000f;  /* Firefox Red 70 */
}

Screenshot is with a live boot of Debian XFCE changed to Adwaita theme, with a clean Firefox 57 profile.