This commit makes the
[color-scheme-toggle](https://github.com/discourse/discourse-color-scheme-toggle)
theme component a core feature with improvements and bug fixes. The
theme component will be updated to become a no-op if the core feature is
enabled.
Noteworthy changes:
* the color mode selector has a new "Auto" option that makes the site
render in the same color mode as the user's system preference
* the splash screen respects the color mode selected by the user
* dark/light variants of category logos and background images are now
picked correctly based on the selected color mode
* a new `interface_color_selector` site setting to disable the selector
or choose its location between the sidebar footer or header
Internal topic: t/139465.
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Co-authored-by: Ella <ella.estigoy@gmail.com>
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