Not sure, you will have to browse through it. Or even pinch some code out. https://github.com/Aris-t2/CustomCSSforFx/issues/2

Looks like this code is used currently:

• chrome://browser/skin/browser.css

You can try to override the current fill color and make sure to add the !important flag.

• fill: #xxxxxx !important;

#downloads-button[attention="success"] > #downloads-indicator-anchor > #downloads-indicator-icon,
#downloads-button[attention="success"] > #downloads-indicator-anchor > #downloads-indicator-progress-outer {
  -moz-context-properties: fill, fill-opacity;
  fill: var(--toolbarbutton-icon-fill-attention);
  fill-opacity: 1;
}

Thanks for your help and directions, I will try and see if I can come up with something.

Edit:

After some trying, I was finally able to get pretty much what I wanted, tanks to the help I got and direction :)

I will share the code, if someone else want to do the same in the future. The current colors are just for prototyping, and should be changed :)

I have currently not updated the colors for when an error occurs during the downloading, but it should be possible to do using the links provided by the other messages.

There might be some unnecessary parts, but it works, so I will keep it.

#downloads-button[attention="success"] #downloads-indicator-icon {
  display: visible !important;
}

/* During a download, force display of progress bar */
#downloads-button[attention="success"] #downloads-indicator-progress-area 
{
  display: -moz-box !important;
  visibility: visible !important;
}

/* After something have successfully completed downloading, set the color of the download icon*/
#downloads-button[attention="success"] > #downloads-indicator-anchor > #downloads-indicator-icon,
#downloads-button[attention="success"] > #downloads-indicator-anchor > #downloads-indicator-progress-outer
{
  fill: #D200FF !important;
}

/* Sets the color of the animated arrow that appears when a download is starting*/
#downloads-button[notification="start"] > #downloads-indicator-anchor > #downloads-indicator-icon,
#downloads-notification-anchor[notification="start"] > #downloads-indicator-notification {
   fill: #557508 !important;
}

/*Set the color of the download icon as something is downloading*/
#downloads-button[progress] > #downloads-indicator-anchor > #downloads-indicator-icon,
#downloads-button[progress] > #downloads-indicator-anchor > #downloads-indicator-progress-outer {
  fill: #557508 !important;
}

/* Set the color for the progress bar as something is downloading */
#downloads-button > #downloads-indicator-anchor > #downloads-indicator-progress-outer > #downloads-indicator-progress-inner {
  fill: yellow !important;
}

/* The default color of the download button */
#downloads-button,
#downloads-button[indicator="true"]:not([attention="success"]) #downloads-indicator-icon
{
  display: -moz-box !important;
  visibility: visible !important;
  fill: #E86110 !important;
}