Editing font end

We installed kobotoolbox on a server using docker. Everything works fine, and my job is to edit a bit the front-end of the login page :

The thing is, as silly as it sounds, i can’t seem to find the files in order to edit this page (I use WinScp to search the files in the server). I understood that they were supposed to be in the kpi container (according to the git hub page of kobotoolbox). I did find some es6 files but I don’t really understand them and I’m not sure they’re doing anything for the front-end.
Can anyome help me on this ?
Thanks !