How to use an image with variations as a background image

So following this thread, https://xenforo.com/community/threa...ns-as-a-background-image.226257/#post-1713256

I have my less like described here, XF 2.3 - Custom prefixes

But when I try to save I get the error "Syntax error - Template modifications: public:extra.less"

CSS:

.fr-box.fr-basic .fr-element:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%...

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