* Resize progress bar to 4px to match spec, rounding the ends of the bar and its tray
* Define and use a set of CSS variables for the progressbar tray color, bar fill and paused fill
* Use the theme's icon attention color for the main progressbar color - when defined - to match the download indicator in the toolbar
* Use a --download-progress-flare-color variable for the highlight color in the indeterminate download animation, which lets us invert the colors in a selected item.
* Remove exceptions/overrides for windows and osx
* Match colors where possible in Places and about:downloads, ensuring we always have contrast in selected download richlistitems
Differential Revision: https://phabricator.services.mozilla.com/D122543
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