Cool CSS trick I stumbled upon,

html:has(.blah) :is(img, iframe, video, embed) {                       
  filter: hue-rotate(-120deg);                                                
}  

Need a different instance of something to have a slightly different color scheme so you can easily tell the difference? You can just hue shift _everything_ by 120 degrees!

I use this to tell the difference between my "corporate"/"personal" webmail client