As pointed out by @Spage in https://gerrit.wikimedia.org/r/#/c/238090/2/resources/master.less the current way to include background images

background-image: url( search_active.svg );

should use the background-image-svg mixin in core's /resources/src/mediawiki.less/mediawiki.mixins.less , or go all-in and use ResourceLoaderImageModule which automatically supplies the background image for a CSS class.