Seems impossible in Firefox v.136 to change sound button style like before or...

hi, in Firefox v.136 the sound button seems totally reworked and I can't get a right way to style that button, or maybe my knowledge is insignificant. I tried this way but works only for English language, I guess I should add an entry for each language ans that seems inefficient:

    
button[aria-label="Mute tab"] .button-background {
        /* my code */
}


  

In the firefox code they use ::part(button) and that don't works in userChrome.css:

    
.tab-audio-button {
  #tabbrowser-tabs:is([orient="vertical"][expanded], [orient="horizontal"]) &:not([pinned]):not([crashed]) {
    &[soundplaying]::part(button) {
   }
  }
}


  

There is a way to solve this or it is impossible?