Hi Aleksandr,
Are you referring to the photo popup background color(http://take.ms/eLwnZ)? If yes, you can try custom css overrides by using the custom css below to modify the color('transparent').

#es.es-photo-popup .es-photo-content {

    background: transparent none repeat scroll 0 0 !important;

}

Your advice does not work (((

Can you kindly please provide us with a sample link and also the login to test this out?

Already entered access to the site!

Hey Aleksandr,
Can you try changing the 'transparent' string in the code to a color of your choice and see if it works?
http://take.ms/gq1AO

1. Prompt, please, what file you changed and on what path it is.
2. Please, tell me what changes need to be made to the code so that the background is transparent?

You may edit the file /templates/rt_topaz/html/com_easysocial/css/custom.css and locate the codes below:

[gist]
#es.es-photo-popup .es-photo-content {
background: transparent none repeat scroll 0 0 !important;
}
[/gist]

If you want it to be white, you could change it to something like this

[gist]
#es.es-photo-popup .es-photo-content {
background: #fff !important;
}
[/gist]

Tell me, please, how to change the background on the images in the pop-up window?

Hey Aleksandr,
We have provided you the custom css codes to change it in our previous reply. Is that not working for you? What do you want to change the background into?