Thunderbird / Mozilla Themes
Images are under creative commons 2.0: https://creativecommons.org/licenses/by/2.0/ and are:
zip solarized_light-1.0-tb.xpi manifest.json images
Dynamic solarized theme which combines light and dark and switches according to real sunrise / sunset times. No idea where to read these times and how to pinpoint user's current location.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Themes https://addons.mozilla.org/en-US/developers/addon/submit/wizard-listed https://github.com/mdn/webextensions-examples/tree/master/dynamic-theme