Add dark mode

[autinerd]

This adds a dark mode for the firmware selector.

[herbetom]

I like the idea of a dark mode.

Hoewer, i don't think this PR can be merged this easily.

It appears to contain a device-pictures submodule? Probably some leftover from a custom fork which probably supports SVGs or PNGs (i. e. pictures without a background)?

With our current use of JPGs this won't look good. :/

Therefore i think making this work would need more significant changes.

[autinerd]

Oh, sorry, I forgot to remove it.

Yes, with the JPEGs (thanks to having no transparency) it looks not so good :/ That's why I switched for our fork at https://firmware.freifunk-stuttgart.de/ to the SVGs from https://github.com/freifunk/device-pictures.

[maurerle]

Also the router.png in this repository needs to be replaced to support transparent background:

[maurerle]

This also depends on #155 for this.
And on #169
and freifunk/device-pictures#35

[maurerle]

@autinerd could you update this PR with the css from ffs - which already has the fix for the footer - please? :)

When selecting "Upgrade" vs "Erstinstallation" the unselected part glows more, which looks as if its is selected, maybe this can be improved as well :)

[autinerd]

I have adjusted the shadow colors as well and added the forgotten color change for the footer :D