I've used Freya before and i can install additional plank themes for example from this site: http://ubuntuhandbook.org/index.php/2015/09/13-plank-themes-elementary-os-freya/ . I tried install that and then right click on the plank with control and i had still four themes. Can I install more themes for Loki? Thanks for help!
plank themes work in loki. Check your .local/share/plank/themes folder. I bet the themes aren't there. It looks like that ppa is down and the one in the comments doesn't support an ubuntu 16.04 based system. The deb link is 404 too. you can download the deb for an older system from: https://launchpad.net/~versable/+archive/ubuntu/elementary-update/+files/elementary-plank-themes_0.4-0~4~precise1_all.deb
open it with archive manager and extract the data file. open the data file in archive manager and extract that too. It will extract to a hidden folder .usr/... Go to the .usr and browse to the plank/themes folder and copy all the themes in that folder to .local/share/plank/themes