[#176] Support dark themes in Firefox

[klappradla]

This addresses a point @bitboxer mentioned in #176: the dark icon may be "too dark" when using a dark theme or macOS' dark mode.

This is a suggestion on how we could support this (at least for Firefox) 🦊 using the theme_icons property in browser_action.
By configuring this, we can show a light menu icon when users use the dark Firefox theme (or macOS "dark mode").

I also - this is only an experiment ⚠️ - changed the colors to match the ones recommended in Firefox' styleguide on iconography 🖌️

The two colors are:

• dark gray: #f9f9facc
• light gray: #0c0c0dcc

This would look like this:

dark 🏴

light 🏳️

The icon changes automatically without adding additional scripts:

NOTE It would also be possible to support this behavior in Chrome. I could only reproduce this via JS and having additional events in the background.js... I personally find the price too high for just the color of the icon for now.

I could - if people are interested - play around with the Chrome approach in a separate PR though. Happy to do that 🤓

[klappradla]

PS: the adaption of the build-icons script was only blindly throwing things in with only half my brain on... We can probably revert most of it 😉

[klappradla]

Done now @pmeinhardt 🧑‍🏭 Had lots of interruptions, sorry 🙇

Million thanks for the review by the way! Really neat points, love the outcome of this 🖤