Suggestions for desaturating an element in Gmail Chat?

[xenornithos]

To preface: I'm on Firefox, using UltimaDark, and referencing the integrated Chat sidebar feature within Gmail.

In Chat, my message bubbles appear as a vivid dark blue, and I'd like to make it grey.

How it currently looks:

How I'd like it to look (as edited in a photo editor, I tweaked the blue section to a Saturation of 0, resulting in the colour hex code of "#272727"):

However, the few attempts I did for affecting the elements (utilising the browser dev tools and "div" elements) has resulted in no visible changes, so I'm not sure what I'm doing improperly or what elements would actually be affected, and would like help on the matter. Any suggestions with how to edit the colour as it appears with UltimaDark applied? I've also heard about a "saturation filter" and I'm not sure if that would be any easier to apply or just to change the colour itself to a grey.

The following two stylesheets are what I've tried thus far with no change in results:

@-moz-document url-prefix("https://mail.google.com/") {
    
/* Colour of my own sent message bubbles */
.z1PNZe {
  background-color: #272727 !important; /* Dark grey */
  color: #000000 !important;
    }
}

@-moz-document url-prefix("https://mail.google.com/") {
    
:root{
    --ud-bg--gm3-color-primary-container:#272727!important;
    }
}

[xenornithos]

Update: I got some answers on how to make it work (as more of an edit to the second stylesheet I previously attempted) -- apparently, I needed to change the domain to "chat.google.com", as that's what is embedded and hence affected in Gmail's sidebar application of it, and the "!important" attribute is unnecessary.

The correct, functional code:

@-moz-document url-prefix("https://chat.google.com/") {
  :root {
    --ud-bg--gm3-color-primary-container: #451010;
  }
}