This PR is the beginning of converting the color picker to use built in
core color palettes.
The `color_schemes` defined in `about.json` are created in core when the
theme is imported,
and we then show all user-selectable color palettes in this sidebar
footer menu.
An important caveat here is that all of the Horizon themes must be
changed to `user_selectable`
otherwise they will not show up in the color palette selector in the
sidebar.
When choosing a color palette that also has a corresponding dark color
palette,
*both* light mode and dark mode are correctly saved with the color
palette(s) chosen,
using the color palette cookie we already have in core.
Anon users can also set a palette, which will be saved in a cookie.
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Co-authored-by: Sérgio Saquetim <saquetim@discourse.org>
Co-authored-by: Martin Brennan <martin@discourse.org>
Co-authored-by: Osama Sayegh <asooomaasoooma90@gmail.com>
Jarek Radosz