Fix: Correct sound tab icon overlay style

[conventoangelo]

Fixes issue 2448.

This pull request includes updates to the src/browser/themes/shared/zen-icons/icons.css file to enhance the visual consistency of tab sound icons.

• Added background-image: none !important; to .tab-icon-overlay[soundplaying], .tab-icon-overlay[muted], and .tab-icon-overlay[activemedia-blocked] to ensure sound icons from Firefox are not displayed.
• Added fill: white; to .tab-icon-overlay[soundplaying], .tab-icon-overlay[muted], and .tab-icon-overlay[activemedia-blocked] to fix sound icon appearance in light mode.

[mr-cheff]

How does it look like in dark mode?

[conventoangelo]

This is how it looks after the change. I also forgot to add !important to the fill property. Would push a new commit in a while.

This is how it looks before: (Notice that when muted, the background image i.e., original Firefox sound icons, can be seen)