So this selector no longer works on mobile using SS 7.1 with their new grid system—I've used the old aforementioned one for a while and it suddenly stopped working. Still works for desktop but not mobile for some reason. Haven't found anyone with a solution so I figured it out myself...eventually. For anyone who stumbles upon this thread try putting this in the page's header code injection:
<style>
/* Invert Mobile Logo */
#header > div.header-announcement-bar-wrapper > div.header-inner.container--fluid.header-mobile-layout-logo-left-nav-right.header-layout-nav-right > div.header-display-mobile > div.header-title-nav-wrapper > div.header-title.header-title--use-mobile-logo.preSlide.slideIn > div.header-mobile-logo > a > picture > img {
-webkit-filter: invert(100%);
filter: invert(100%);
filter:progid:DXImageTransform.Microsoft.BasicImage(invert='1');
}
</style>