Comment 6 for bug 1624738

The border colour can be set with:

frame {
    border-width: 1px;
    border-style: solid;
    border-color: shade (@bg_color, 0.82);
}

But 'frame' is used widely in many applications which results in borders being rendered where you really don't want them. The following will style Firefox and then override that styling where it is not required. I've tested most default applications in Ubuntu and the following appears to work well, more testing would be good.

/* Style frame for firefox to set the search/location entry box borders*/
frame {
    border-width: 1px;
    border-style: solid;
    border-color: shade (@bg_color, 0.82);
}

/* Override frame styling set for firefox to prevent erroneous frames being rendered */
box > frame, /* common in settings panels */
statusbar > frame, /* common in application status bars, etc. */
statusbar > widget > frame, /* common in application status bars, etc. */
GvcMixerDialog frame { /* some audio mixers */
    border-width: 0px;
    border-style: none;
}