Better Browser Dark Mode Support

[hnryjms]

With Firefox and hopefully upcoming Safari support, we have many browsers that include Dark Mode. The current icon doesn't look stellar in dark mode, so we should revisit the symbol and what approach we want to take when the browser has a dark top bar.

• https://discourse.mozilla.org/t/how-to-correctly-design-color-icon-to-be-visible-across-themes/27713/5
• https://developer.apple.com/documentation/safariservices/safari_web_extensions/customizing_a_safari_web_extension#3678666

We also have a UI that renders both a Dark and a Light mode, and we should make sure there's a consistent feel for that style. Additionally, we may want to change the symbol in the header of the bar to match the logo of our web extension.

[1ShadyRabbit]

lets do this

[hnryjms]

Hey @1ShadyRabbit I'd appreciate your help on this. If you're interested in taking a stab at it, just fork this repo and point a PR here once you're done!