Migrate our colors to Acorn

For reference: https://gitlab.torproject.org/tpo/applications/tor-browser/-/blob/tor-browser-102.8.0esr-12.5-1/browser/themes/shared/browser-colors.css

So this started out as photon colors some time back because no-where in Firefox's code (at the time) were all of the photon colors fully defined. We added a single canonical file with all the colors you could want and things were good.

Meanwhile, some time has passed and there are now at least 3 different color systems in use in Firefox:

• photon: https://github.com/FirefoxUX/photon-colors/blob/master/photon-colors.css
• protocol: https://protocol.mozilla.org/docs/fundamentals/color.html
• acorn: https://acorn.firefox.com/latest/styles/color.html

So the UX team is always very nice and gives us hex codes to work with when dealing with colors as well as the css vars (eg --yellow-50 / #ffa436). The problem is that (most recently) the color/name combos seem to coincide with the protocol colors rather than our existing photon colors in css. Obviously hexcode is king but we should all agree on which colour system we're using and update browser-colors.css to the actual system we are using to avoid confusion in the future.