there is nothing in that themes folder
Two possibilities : if YOU have uploaded the image via Oxwall, it should be somewhere in ow_userfiles.
Second one : You go to : Oxwall folder -> ow_static -> themes -> "Your personnal theme" -> images
You can also go to the folder of your personnal theme, find the base.css file and modify the line : background-image:url('.....your image location....') to background:none.
body.ow.base_sign_in{
background-color: none!important;
background-image: none!important
}
just apply these styles in the theme customizer here: www.site.com//admin/theme/css