Kickass Feature: Allow extracting a color scheme from an uploaded image, and apply it to themes supporting color module.

This is part of the #1905 meta...

Describe your issue or idea

This idea has been mentioned in the past by @wesruv while building Basis, and also in an unrelated context in https://github.com/backdrop/backdrop-issues/issues/1224#issuecomment-141...

There are php libraries that could help with this:

• https://github.com/pedrosancao/php-color-extractor
• https://github.com/thephpleague/color-extractor
• https://github.com/bachors/PHP-Extract-Image-Color-Class
• ...moar??

Expected behavior (if reporting a bug)

When users visit the settings page of a theme that has color.module support, they are presented with a button to upload their favourite image or their company logo. Once uploaded, we extract a color scheme and apply it to the theme. The user can now preview and tweak.